Btrfs: get write access when setting the default subvolume

[linux-2.6.git] / fs / buffer.c

/*
 *  linux/fs/buffer.c
 *
 *  Copyright (C) 1991, 1992, 2002  Linus Torvalds
 */

/*
 * Start bdflush() with kernel_thread not syscall - Paul Gortmaker, 12/95
 *
 * Removed a lot of unnecessary code and simplified things now that
 * the buffer cache isn't our primary cache - Andrew Tridgell 12/96
 *
 * Speed up hash, lru, and free list operations.  Use gfp() for allocating
 * hash table, use SLAB cache for buffer heads. SMP threading.  -DaveM
 *
 * Added 32k buffer block sizes - these are required older ARM systems. - RMK
 *
 * async buffer flushing, 1999 Andrea Arcangeli <andrea@suse.de>
 */

#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/percpu.h>
#include <linux/slab.h>
#include <linux/capability.h>
#include <linux/blkdev.h>
#include <linux/file.h>
#include <linux/quotaops.h>
#include <linux/highmem.h>
#include <linux/export.h>
#include <linux/writeback.h>
#include <linux/hash.h>
#include <linux/suspend.h>
#include <linux/buffer_head.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/bio.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/bitops.h>
#include <linux/mpage.h>
#include <linux/bit_spinlock.h>

static int fsync_buffers_list(spinlock_t *lock, struct list_head *list);

#define BH_ENTRY(list) list_entry((list), struct buffer_head, b_assoc_buffers)

inline void
init_buffer(struct buffer_head *bh, bh_end_io_t *handler, void *private)
{
        bh->b_end_io = handler;
        bh->b_private = private;
}
EXPORT_SYMBOL(init_buffer);

static int sleep_on_buffer(void *word)
{
        io_schedule();
        return 0;
}

void __lock_buffer(struct buffer_head *bh)
{
        wait_on_bit_lock(&bh->b_state, BH_Lock, sleep_on_buffer,
                                                        TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__lock_buffer);

void unlock_buffer(struct buffer_head *bh)
{
        clear_bit_unlock(BH_Lock, &bh->b_state);
        smp_mb__after_clear_bit();
        wake_up_bit(&bh->b_state, BH_Lock);
}
EXPORT_SYMBOL(unlock_buffer);

/*
 * Block until a buffer comes unlocked.  This doesn't stop it
 * from becoming locked again - you have to lock it yourself
 * if you want to preserve its state.
 */
void __wait_on_buffer(struct buffer_head * bh)
{
        wait_on_bit(&bh->b_state, BH_Lock, sleep_on_buffer, TASK_UNINTERRUPTIBLE);
}
EXPORT_SYMBOL(__wait_on_buffer);

static void
__clear_page_buffers(struct page *page)
{
        ClearPagePrivate(page);
        set_page_private(page, 0);
        page_cache_release(page);
}


static int quiet_error(struct buffer_head *bh)
{
        if (!test_bit(BH_Quiet, &bh->b_state) && printk_ratelimit())
                return 0;
        return 1;
}


static void buffer_io_error(struct buffer_head *bh)
{
        char b[BDEVNAME_SIZE];
        printk(KERN_ERR "Buffer I/O error on device %s, logical block %Lu\n",
                        bdevname(bh->b_bdev, b),
                        (unsigned long long)bh->b_blocknr);
}

/*
 * End-of-IO handler helper function which does not touch the bh after
 * unlocking it.
 * Note: unlock_buffer() sort-of does touch the bh after unlocking it, but
 * a race there is benign: unlock_buffer() only use the bh's address for
 * hashing after unlocking the buffer, so it doesn't actually touch the bh
 * itself.
 */
static void __end_buffer_read_notouch(struct buffer_head *bh, int uptodate)
{
        if (uptodate) {
                set_buffer_uptodate(bh);
        } else {
                /* This happens, due to failed READA attempts. */
                clear_buffer_uptodate(bh);
        }
        unlock_buffer(bh);
}

/*
 * Default synchronous end-of-IO handler..  Just mark it up-to-date and
 * unlock the buffer. This is what ll_rw_block uses too.
 */
void end_buffer_read_sync(struct buffer_head *bh, int uptodate)
{
        __end_buffer_read_notouch(bh, uptodate);
        put_bh(bh);
}
EXPORT_SYMBOL(end_buffer_read_sync);

void end_buffer_write_sync(struct buffer_head *bh, int uptodate)
{
        char b[BDEVNAME_SIZE];

        if (uptodate) {
                set_buffer_uptodate(bh);
        } else {
                if (!quiet_error(bh)) {
                        buffer_io_error(bh);
                        printk(KERN_WARNING "lost page write due to "
                                        "I/O error on %s\n",
                                       bdevname(bh->b_bdev, b));
                }
                set_buffer_write_io_error(bh);
                clear_buffer_uptodate(bh);
        }
        unlock_buffer(bh);
        put_bh(bh);
}
EXPORT_SYMBOL(end_buffer_write_sync);

/*
 * Various filesystems appear to want __find_get_block to be non-blocking.
 * But it's the page lock which protects the buffers.  To get around this,
 * we get exclusion from try_to_free_buffers with the blockdev mapping's
 * private_lock.
 *
 * Hack idea: for the blockdev mapping, i_bufferlist_lock contention
 * may be quite high.  This code could TryLock the page, and if that
 * succeeds, there is no need to take private_lock. (But if
 * private_lock is contended then so is mapping->tree_lock).
 */
static struct buffer_head *
__find_get_block_slow(struct block_device *bdev, sector_t block)
{
        struct inode *bd_inode = bdev->bd_inode;
        struct address_space *bd_mapping = bd_inode->i_mapping;
        struct buffer_head *ret = NULL;
        pgoff_t index;
        struct buffer_head *bh;
        struct buffer_head *head;
        struct page *page;
        int all_mapped = 1;

        index = block >> (PAGE_CACHE_SHIFT - bd_inode->i_blkbits);
        page = find_get_page(bd_mapping, index);
        if (!page)
                goto out;

        spin_lock(&bd_mapping->private_lock);
        if (!page_has_buffers(page))
                goto out_unlock;
        head = page_buffers(page);
        bh = head;
        do {
                if (!buffer_mapped(bh))
                        all_mapped = 0;
                else if (bh->b_blocknr == block) {
                        ret = bh;
                        get_bh(bh);
                        goto out_unlock;
                }
                bh = bh->b_this_page;
        } while (bh != head);

        /* we might be here because some of the buffers on this page are
         * not mapped.  This is due to various races between
         * file io on the block device and getblk.  It gets dealt with
         * elsewhere, don't buffer_error if we had some unmapped buffers
         */
        if (all_mapped) {
                char b[BDEVNAME_SIZE];

                printk("__find_get_block_slow() failed. "
                        "block=%llu, b_blocknr=%llu\n",
                        (unsigned long long)block,
                        (unsigned long long)bh->b_blocknr);
                printk("b_state=0x%08lx, b_size=%zu\n",
                        bh->b_state, bh->b_size);
                printk("device %s blocksize: %d\n", bdevname(bdev, b),
                        1 << bd_inode->i_blkbits);
        }
out_unlock:
        spin_unlock(&bd_mapping->private_lock);
        page_cache_release(page);
out:
        return ret;
}

/*
 * Kick the writeback threads then try to free up some ZONE_NORMAL memory.
 */
static void free_more_memory(void)
{
        struct zone *zone;
        int nid;

        wakeup_flusher_threads(1024, WB_REASON_FREE_MORE_MEM);
        yield();

        for_each_online_node(nid) {
                (void)first_zones_zonelist(node_zonelist(nid, GFP_NOFS),
                                                gfp_zone(GFP_NOFS), NULL,
                                                &zone);
                if (zone)
                        try_to_free_pages(node_zonelist(nid, GFP_NOFS), 0,
                                                GFP_NOFS, NULL);
        }
}

/*
 * I/O completion handler for block_read_full_page() - pages
 * which come unlocked at the end of I/O.
 */
static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
{
        unsigned long flags;
        struct buffer_head *first;
        struct buffer_head *tmp;
        struct page *page;
        int page_uptodate = 1;

        BUG_ON(!buffer_async_read(bh));

        page = bh->b_page;
        if (uptodate) {
                set_buffer_uptodate(bh);
        } else {
                clear_buffer_uptodate(bh);
                if (!quiet_error(bh))
                        buffer_io_error(bh);
                SetPageError(page);
        }

        /*
         * Be _very_ careful from here on. Bad things can happen if
         * two buffer heads end IO at almost the same time and both
         * decide that the page is now completely done.
         */
        first = page_buffers(page);
        local_irq_save(flags);
        bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
        clear_buffer_async_read(bh);
        unlock_buffer(bh);
        tmp = bh;
        do {
                if (!buffer_uptodate(tmp))
                        page_uptodate = 0;
                if (buffer_async_read(tmp)) {
                        BUG_ON(!buffer_locked(tmp));
                        goto still_busy;
                }
                tmp = tmp->b_this_page;
        } while (tmp != bh);
        bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
        local_irq_restore(flags);

        /*
         * If none of the buffers had errors and they are all
         * uptodate then we can set the page uptodate.
         */
        if (page_uptodate && !PageError(page))
                SetPageUptodate(page);
        unlock_page(page);
        return;

still_busy:
        bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
        local_irq_restore(flags);
        return;
}

/*
 * Completion handler for block_write_full_page() - pages which are unlocked
 * during I/O, and which have PageWriteback cleared upon I/O completion.
 */
void end_buffer_async_write(struct buffer_head *bh, int uptodate)
{
        char b[BDEVNAME_SIZE];
        unsigned long flags;
        struct buffer_head *first;
        struct buffer_head *tmp;
        struct page *page;

        BUG_ON(!buffer_async_write(bh));

        page = bh->b_page;
        if (uptodate) {
                set_buffer_uptodate(bh);
        } else {
                if (!quiet_error(bh)) {
                        buffer_io_error(bh);
                        printk(KERN_WARNING "lost page write due to "
                                        "I/O error on %s\n",
                               bdevname(bh->b_bdev, b));
                }
                set_bit(AS_EIO, &page->mapping->flags);
                set_buffer_write_io_error(bh);
                clear_buffer_uptodate(bh);
                SetPageError(page);
        }

        first = page_buffers(page);
        local_irq_save(flags);
        bit_spin_lock(BH_Uptodate_Lock, &first->b_state);

        clear_buffer_async_write(bh);
        unlock_buffer(bh);
        tmp = bh->b_this_page;
        while (tmp != bh) {
                if (buffer_async_write(tmp)) {
                        BUG_ON(!buffer_locked(tmp));
                        goto still_busy;
                }
                tmp = tmp->b_this_page;
        }
        bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
        local_irq_restore(flags);
        end_page_writeback(page);
        return;

still_busy:
        bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
        local_irq_restore(flags);
        return;
}
EXPORT_SYMBOL(end_buffer_async_write);

/*
 * If a page's buffers are under async readin (end_buffer_async_read
 * completion) then there is a possibility that another thread of
 * control could lock one of the buffers after it has completed
 * but while some of the other buffers have not completed.  This
 * locked buffer would confuse end_buffer_async_read() into not unlocking
 * the page.  So the absence of BH_Async_Read tells end_buffer_async_read()
 * that this buffer is not under async I/O.
 *
 * The page comes unlocked when it has no locked buffer_async buffers
 * left.
 *
 * PageLocked prevents anyone starting new async I/O reads any of
 * the buffers.
 *
 * PageWriteback is used to prevent simultaneous writeout of the same
 * page.
 *
 * PageLocked prevents anyone from starting writeback of a page which is
 * under read I/O (PageWriteback is only ever set against a locked page).
 */
static void mark_buffer_async_read(struct buffer_head *bh)
{
        bh->b_end_io = end_buffer_async_read;
        set_buffer_async_read(bh);
}

static void mark_buffer_async_write_endio(struct buffer_head *bh,
                                          bh_end_io_t *handler)
{
        bh->b_end_io = handler;
        set_buffer_async_write(bh);
}

void mark_buffer_async_write(struct buffer_head *bh)
{
        mark_buffer_async_write_endio(bh, end_buffer_async_write);
}
EXPORT_SYMBOL(mark_buffer_async_write);


/*
 * fs/buffer.c contains helper functions for buffer-backed address space's
 * fsync functions.  A common requirement for buffer-based filesystems is
 * that certain data from the backing blockdev needs to be written out for
 * a successful fsync().  For example, ext2 indirect blocks need to be
 * written back and waited upon before fsync() returns.
 *
 * The functions mark_buffer_inode_dirty(), fsync_inode_buffers(),
 * inode_has_buffers() and invalidate_inode_buffers() are provided for the
 * management of a list of dependent buffers at ->i_mapping->private_list.
 *
 * Locking is a little subtle: try_to_free_buffers() will remove buffers
 * from their controlling inode's queue when they are being freed.  But
 * try_to_free_buffers() will be operating against the *blockdev* mapping
 * at the time, not against the S_ISREG file which depends on those buffers.
 * So the locking for private_list is via the private_lock in the address_space
 * which backs the buffers.  Which is different from the address_space 
 * against which the buffers are listed.  So for a particular address_space,
 * mapping->private_lock does *not* protect mapping->private_list!  In fact,
 * mapping->private_list will always be protected by the backing blockdev's
 * ->private_lock.
 *
 * Which introduces a requirement: all buffers on an address_space's
 * ->private_list must be from the same address_space: the blockdev's.
 *
 * address_spaces which do not place buffers at ->private_list via these
 * utility functions are free to use private_lock and private_list for
 * whatever they want.  The only requirement is that list_empty(private_list)
 * be true at clear_inode() time.
 *
 * FIXME: clear_inode should not call invalidate_inode_buffers().  The
 * filesystems should do that.  invalidate_inode_buffers() should just go
 * BUG_ON(!list_empty).
 *
 * FIXME: mark_buffer_dirty_inode() is a data-plane operation.  It should
 * take an address_space, not an inode.  And it should be called
 * mark_buffer_dirty_fsync() to clearly define why those buffers are being
 * queued up.
 *
 * FIXME: mark_buffer_dirty_inode() doesn't need to add the buffer to the
 * list if it is already on a list.  Because if the buffer is on a list,
 * it *must* already be on the right one.  If not, the filesystem is being
 * silly.  This will save a ton of locking.  But first we have to ensure
 * that buffers are taken *off* the old inode's list when they are freed
 * (presumably in truncate).  That requires careful auditing of all
 * filesystems (do it inside bforget()).  It could also be done by bringing
 * b_inode back.
 */

/*
 * The buffer's backing address_space's private_lock must be held
 */
static void __remove_assoc_queue(struct buffer_head *bh)
{
        list_del_init(&bh->b_assoc_buffers);
        WARN_ON(!bh->b_assoc_map);
        if (buffer_write_io_error(bh))
                set_bit(AS_EIO, &bh->b_assoc_map->flags);
        bh->b_assoc_map = NULL;
}

int inode_has_buffers(struct inode *inode)
{
        return !list_empty(&inode->i_data.private_list);
}

/*
 * osync is designed to support O_SYNC io.  It waits synchronously for
 * all already-submitted IO to complete, but does not queue any new
 * writes to the disk.
 *
 * To do O_SYNC writes, just queue the buffer writes with ll_rw_block as
 * you dirty the buffers, and then use osync_inode_buffers to wait for
 * completion.  Any other dirty buffers which are not yet queued for
 * write will not be flushed to disk by the osync.
 */
static int osync_buffers_list(spinlock_t *lock, struct list_head *list)
{
        struct buffer_head *bh;
        struct list_head *p;
        int err = 0;

        spin_lock(lock);
repeat:
        list_for_each_prev(p, list) {
                bh = BH_ENTRY(p);
                if (buffer_locked(bh)) {
                        get_bh(bh);
                        spin_unlock(lock);
                        wait_on_buffer(bh);
                        if (!buffer_uptodate(bh))
                                err = -EIO;
                        brelse(bh);
                        spin_lock(lock);
                        goto repeat;
                }
        }
        spin_unlock(lock);
        return err;
}

static void do_thaw_one(struct super_block *sb, void *unused)
{
        char b[BDEVNAME_SIZE];
        while (sb->s_bdev && !thaw_bdev(sb->s_bdev, sb))
                printk(KERN_WARNING "Emergency Thaw on %s\n",
                       bdevname(sb->s_bdev, b));
}

static void do_thaw_all(struct work_struct *work)
{
        iterate_supers(do_thaw_one, NULL);
        kfree(work);
        printk(KERN_WARNING "Emergency Thaw complete\n");
}

/**
 * emergency_thaw_all -- forcibly thaw every frozen filesystem
 *
 * Used for emergency unfreeze of all filesystems via SysRq
 */
void emergency_thaw_all(void)
{
        struct work_struct *work;

        work = kmalloc(sizeof(*work), GFP_ATOMIC);
        if (work) {
                INIT_WORK(work, do_thaw_all);
                schedule_work(work);
        }
}

/**
 * sync_mapping_buffers - write out & wait upon a mapping's "associated" buffers
 * @mapping: the mapping which wants those buffers written
 *
 * Starts I/O against the buffers at mapping->private_list, and waits upon
 * that I/O.
 *
 * Basically, this is a convenience function for fsync().
 * @mapping is a file or directory which needs those buffers to be written for
 * a successful fsync().
 */
int sync_mapping_buffers(struct address_space *mapping)
{
        struct address_space *buffer_mapping = mapping->assoc_mapping;

        if (buffer_mapping == NULL || list_empty(&mapping->private_list))
                return 0;

        return fsync_buffers_list(&buffer_mapping->private_lock,
                                        &mapping->private_list);
}
EXPORT_SYMBOL(sync_mapping_buffers);

/*
 * Called when we've recently written block `bblock', and it is known that
 * `bblock' was for a buffer_boundary() buffer.  This means that the block at
 * `bblock + 1' is probably a dirty indirect block.  Hunt it down and, if it's
 * dirty, schedule it for IO.  So that indirects merge nicely with their data.
 */
void write_boundary_block(struct block_device *bdev,
                        sector_t bblock, unsigned blocksize)
{
        struct buffer_head *bh = __find_get_block(bdev, bblock + 1, blocksize);
        if (bh) {
                if (buffer_dirty(bh))
                        ll_rw_block(WRITE, 1, &bh);
                put_bh(bh);
        }
}

void mark_buffer_dirty_inode(struct buffer_head *bh, struct inode *inode)
{
        struct address_space *mapping = inode->i_mapping;
        struct address_space *buffer_mapping = bh->b_page->mapping;

        mark_buffer_dirty(bh);
        if (!mapping->assoc_mapping) {
                mapping->assoc_mapping = buffer_mapping;
        } else {
                BUG_ON(mapping->assoc_mapping != buffer_mapping);
        }
        if (!bh->b_assoc_map) {
                spin_lock(&buffer_mapping->private_lock);
                list_move_tail(&bh->b_assoc_buffers,
                                &mapping->private_list);
                bh->b_assoc_map = mapping;
                spin_unlock(&buffer_mapping->private_lock);
        }
}
EXPORT_SYMBOL(mark_buffer_dirty_inode);

/*
 * Mark the page dirty, and set it dirty in the radix tree, and mark the inode
 * dirty.
 *
 * If warn is true, then emit a warning if the page is not uptodate and has
 * not been truncated.
 */
static void __set_page_dirty(struct page *page,
                struct address_space *mapping, int warn)
{
        spin_lock_irq(&mapping->tree_lock);
        if (page->mapping) {    /* Race with truncate? */
                WARN_ON_ONCE(warn && !PageUptodate(page));
                account_page_dirtied(page, mapping);
                radix_tree_tag_set(&mapping->page_tree,
                                page_index(page), PAGECACHE_TAG_DIRTY);
        }
        spin_unlock_irq(&mapping->tree_lock);
        __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
}

/*
 * Add a page to the dirty page list.
 *
 * It is a sad fact of life that this function is called from several places
 * deeply under spinlocking.  It may not sleep.
 *
 * If the page has buffers, the uptodate buffers are set dirty, to preserve
 * dirty-state coherency between the page and the buffers.  It the page does
 * not have buffers then when they are later attached they will all be set
 * dirty.
 *
 * The buffers are dirtied before the page is dirtied.  There's a small race
 * window in which a writepage caller may see the page cleanness but not the
 * buffer dirtiness.  That's fine.  If this code were to set the page dirty
 * before the buffers, a concurrent writepage caller could clear the page dirty
 * bit, see a bunch of clean buffers and we'd end up with dirty buffers/clean
 * page on the dirty page list.
 *
 * We use private_lock to lock against try_to_free_buffers while using the
 * page's buffer list.  Also use this to protect against clean buffers being
 * added to the page after it was set dirty.
 *
 * FIXME: may need to call ->reservepage here as well.  That's rather up to the
 * address_space though.
 */
int __set_page_dirty_buffers(struct page *page)
{
        int newly_dirty;
        struct address_space *mapping = page_mapping(page);

        if (unlikely(!mapping))
                return !TestSetPageDirty(page);

        spin_lock(&mapping->private_lock);
        if (page_has_buffers(page)) {
                struct buffer_head *head = page_buffers(page);
                struct buffer_head *bh = head;

                do {
                        set_buffer_dirty(bh);
                        bh = bh->b_this_page;
                } while (bh != head);
        }
        newly_dirty = !TestSetPageDirty(page);
        spin_unlock(&mapping->private_lock);

        if (newly_dirty)
                __set_page_dirty(page, mapping, 1);
        return newly_dirty;
}
EXPORT_SYMBOL(__set_page_dirty_buffers);

/*
 * Write out and wait upon a list of buffers.
 *
 * We have conflicting pressures: we want to make sure that all
 * initially dirty buffers get waited on, but that any subsequently
 * dirtied buffers don't.  After all, we don't want fsync to last
 * forever if somebody is actively writing to the file.
 *
 * Do this in two main stages: first we copy dirty buffers to a
 * temporary inode list, queueing the writes as we go.  Then we clean
 * up, waiting for those writes to complete.
 * 
 * During this second stage, any subsequent updates to the file may end
 * up refiling the buffer on the original inode's dirty list again, so
 * there is a chance we will end up with a buffer queued for write but
 * not yet completed on that list.  So, as a final cleanup we go through
 * the osync code to catch these locked, dirty buffers without requeuing
 * any newly dirty buffers for write.
 */
static int fsync_buffers_list(spinlock_t *lock, struct list_head *list)
{
        struct buffer_head *bh;
        struct list_head tmp;
        struct address_space *mapping;
        int err = 0, err2;
        struct blk_plug plug;

        INIT_LIST_HEAD(&tmp);
        blk_start_plug(&plug);

        spin_lock(lock);
        while (!list_empty(list)) {
                bh = BH_ENTRY(list->next);
                mapping = bh->b_assoc_map;
                __remove_assoc_queue(bh);
                /* Avoid race with mark_buffer_dirty_inode() which does
                 * a lockless check and we rely on seeing the dirty bit */
                smp_mb();
                if (buffer_dirty(bh) || buffer_locked(bh)) {
                        list_add(&bh->b_assoc_buffers, &tmp);
                        bh->b_assoc_map = mapping;
                        if (buffer_dirty(bh)) {
                                get_bh(bh);
                                spin_unlock(lock);
                                /*
                                 * Ensure any pending I/O completes so that
                                 * write_dirty_buffer() actually writes the
                                 * current contents - it is a noop if I/O is
                                 * still in flight on potentially older
                                 * contents.
                                 */
                                write_dirty_buffer(bh, WRITE_SYNC);

                                /*
                                 * Kick off IO for the previous mapping. Note
                                 * that we will not run the very last mapping,
                                 * wait_on_buffer() will do that for us
                                 * through sync_buffer().
                                 */
                                brelse(bh);
                                spin_lock(lock);
                        }
                }
        }

        spin_unlock(lock);
        blk_finish_plug(&plug);
        spin_lock(lock);

        while (!list_empty(&tmp)) {
                bh = BH_ENTRY(tmp.prev);
                get_bh(bh);
                mapping = bh->b_assoc_map;
                __remove_assoc_queue(bh);
                /* Avoid race with mark_buffer_dirty_inode() which does
                 * a lockless check and we rely on seeing the dirty bit */
                smp_mb();
                if (buffer_dirty(bh)) {
                        list_add(&bh->b_assoc_buffers,
                                 &mapping->private_list);
                        bh->b_assoc_map = mapping;
                }
                spin_unlock(lock);
                wait_on_buffer(bh);
                if (!buffer_uptodate(bh))
                        err = -EIO;
                brelse(bh);
                spin_lock(lock);
        }
        
        spin_unlock(lock);
        err2 = osync_buffers_list(lock, list);
        if (err)
                return err;
        else
                return err2;
}

/*
 * Invalidate any and all dirty buffers on a given inode.  We are
 * probably unmounting the fs, but that doesn't mean we have already
 * done a sync().  Just drop the buffers from the inode list.
 *
 * NOTE: we take the inode's blockdev's mapping's private_lock.  Which
 * assumes that all the buffers are against the blockdev.  Not true
 * for reiserfs.
 */
void invalidate_inode_buffers(struct inode *inode)
{
        if (inode_has_buffers(inode)) {
                struct address_space *mapping = &inode->i_data;
                struct list_head *list = &mapping->private_list;
                struct address_space *buffer_mapping = mapping->assoc_mapping;

                spin_lock(&buffer_mapping->private_lock);
                while (!list_empty(list))
                        __remove_assoc_queue(BH_ENTRY(list->next));
                spin_unlock(&buffer_mapping->private_lock);
        }
}
EXPORT_SYMBOL(invalidate_inode_buffers);

/*
 * Remove any clean buffers from the inode's buffer list.  This is called
 * when we're trying to free the inode itself.  Those buffers can pin it.
 *
 * Returns true if all buffers were removed.
 */
int remove_inode_buffers(struct inode *inode)
{
        int ret = 1;

        if (inode_has_buffers(inode)) {
                struct address_space *mapping = &inode->i_data;
                struct list_head *list = &mapping->private_list;
                struct address_space *buffer_mapping = mapping->assoc_mapping;

                spin_lock(&buffer_mapping->private_lock);
                while (!list_empty(list)) {
                        struct buffer_head *bh = BH_ENTRY(list->next);
                        if (buffer_dirty(bh)) {
                                ret = 0;
                                break;
                        }
                        __remove_assoc_queue(bh);
                }
                spin_unlock(&buffer_mapping->private_lock);
        }
        return ret;
}

/*
 * Create the appropriate buffers when given a page for data area and
 * the size of each buffer.. Use the bh->b_this_page linked list to
 * follow the buffers created.  Return NULL if unable to create more
 * buffers.
 *
 * The retry flag is used to differentiate async IO (paging, swapping)
 * which may not fail from ordinary buffer allocations.
 */
struct buffer_head *alloc_page_buffers(struct page *page, unsigned long size,
                int retry)
{
        struct buffer_head *bh, *head;
        long offset;

try_again:
        head = NULL;
        offset = PAGE_SIZE;
        while ((offset -= size) >= 0) {
                bh = alloc_buffer_head(GFP_NOFS);
                if (!bh)
                        goto no_grow;

                bh->b_bdev = NULL;
                bh->b_this_page = head;
                bh->b_blocknr = -1;
                head = bh;

                bh->b_state = 0;
                atomic_set(&bh->b_count, 0);
                bh->b_size = size;

                /* Link the buffer to its page */
                set_bh_page(bh, page, offset);

                init_buffer(bh, NULL, NULL);
        }
        return head;
/*
 * In case anything failed, we just free everything we got.
 */
no_grow:
        if (head) {
                do {
                        bh = head;
                        head = head->b_this_page;
                        free_buffer_head(bh);
                } while (head);
        }

        /*
         * Return failure for non-async IO requests.  Async IO requests
         * are not allowed to fail, so we have to wait until buffer heads
         * become available.  But we don't want tasks sleeping with 
         * partially complete buffers, so all were released above.
         */
        if (!retry)
                return NULL;

        /* We're _really_ low on memory. Now we just
         * wait for old buffer heads to become free due to
         * finishing IO.  Since this is an async request and
         * the reserve list is empty, we're sure there are 
         * async buffer heads in use.
         */
        free_more_memory();
        goto try_again;
}
EXPORT_SYMBOL_GPL(alloc_page_buffers);

static inline void
link_dev_buffers(struct page *page, struct buffer_head *head)
{
        struct buffer_head *bh, *tail;

        bh = head;
        do {
                tail = bh;
                bh = bh->b_this_page;
        } while (bh);
        tail->b_this_page = head;
        attach_page_buffers(page, head);
}

static sector_t blkdev_max_block(struct block_device *bdev, unsigned int size)
{
        sector_t retval = ~((sector_t)0);
        loff_t sz = i_size_read(bdev->bd_inode);

        if (sz) {
                unsigned int sizebits = blksize_bits(size);
                retval = (sz >> sizebits);
        }
        return retval;
}

/*
 * Initialise the state of a blockdev page's buffers.
 */ 
static sector_t
init_page_buffers(struct page *page, struct block_device *bdev,
                        sector_t block, int size)
{
        struct buffer_head *head = page_buffers(page);
        struct buffer_head *bh = head;
        int uptodate = PageUptodate(page);
        sector_t end_block = blkdev_max_block(I_BDEV(bdev->bd_inode), size);

        do {
                if (!buffer_mapped(bh)) {
                        init_buffer(bh, NULL, NULL);
                        bh->b_bdev = bdev;
                        bh->b_blocknr = block;
                        if (uptodate)
                                set_buffer_uptodate(bh);
                        if (block < end_block)
                                set_buffer_mapped(bh);
                }
                block++;
                bh = bh->b_this_page;
        } while (bh != head);

        /*
         * Caller needs to validate requested block against end of device.
         */
        return end_block;
}

/*
 * Create the page-cache page that contains the requested block.
 *
 * This is used purely for blockdev mappings.
 */
static int
grow_dev_page(struct block_device *bdev, sector_t block,
                pgoff_t index, int size, int sizebits)
{
        struct inode *inode = bdev->bd_inode;
        struct page *page;
        struct buffer_head *bh;
        sector_t end_block;
        int ret = 0;            /* Will call free_more_memory() */

        page = find_or_create_page(inode->i_mapping, index,
                (mapping_gfp_mask(inode->i_mapping) & ~__GFP_FS)|__GFP_MOVABLE);
        if (!page)
                return ret;

        BUG_ON(!PageLocked(page));

        if (page_has_buffers(page)) {
                bh = page_buffers(page);
                if (bh->b_size == size) {
                        end_block = init_page_buffers(page, bdev,
                                                index << sizebits, size);
                        goto done;
                }
                if (!try_to_free_buffers(page))
                        goto failed;
        }

        /*
         * Allocate some buffers for this page
         */
        bh = alloc_page_buffers(page, size, 0);
        if (!bh)
                goto failed;

        /*
         * Link the page to the buffers and initialise them.  Take the
         * lock to be atomic wrt __find_get_block(), which does not
         * run under the page lock.
         */
        spin_lock(&inode->i_mapping->private_lock);
        link_dev_buffers(page, bh);
        end_block = init_page_buffers(page, bdev, index << sizebits, size);
        spin_unlock(&inode->i_mapping->private_lock);
done:
        ret = (block < end_block) ? 1 : -ENXIO;
failed:
        unlock_page(page);
        page_cache_release(page);
        return ret;
}

/*
 * Create buffers for the specified block device block's page.  If
 * that page was dirty, the buffers are set dirty also.
 */
static int
grow_buffers(struct block_device *bdev, sector_t block, int size)
{
        pgoff_t index;
        int sizebits;

        sizebits = -1;
        do {
                sizebits++;
        } while ((size << sizebits) < PAGE_SIZE);

        index = block >> sizebits;

        /*
         * Check for a block which wants to lie outside our maximum possible
         * pagecache index.  (this comparison is done using sector_t types).
         */
        if (unlikely(index != block >> sizebits)) {
                char b[BDEVNAME_SIZE];

                printk(KERN_ERR "%s: requested out-of-range block %llu for "
                        "device %s\n",
                        __func__, (unsigned long long)block,
                        bdevname(bdev, b));
                return -EIO;
        }

        /* Create a page with the proper size buffers.. */
        return grow_dev_page(bdev, block, index, size, sizebits);
}

static struct buffer_head *
__getblk_slow(struct block_device *bdev, sector_t block, int size)
{
        /* Size must be multiple of hard sectorsize */
        if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
                        (size < 512 || size > PAGE_SIZE))) {
                printk(KERN_ERR "getblk(): invalid block size %d requested\n",
                                        size);
                printk(KERN_ERR "logical block size: %d\n",
                                        bdev_logical_block_size(bdev));

                dump_stack();
                return NULL;
        }

        for (;;) {
                struct buffer_head *bh;
                int ret;

                bh = __find_get_block(bdev, block, size);
                if (bh)
                        return bh;

                ret = grow_buffers(bdev, block, size);
                if (ret < 0)
                        return NULL;
                if (ret == 0)
                        free_more_memory();
        }
}

/*
 * The relationship between dirty buffers and dirty pages:
 *
 * Whenever a page has any dirty buffers, the page's dirty bit is set, and
 * the page is tagged dirty in its radix tree.
 *
 * At all times, the dirtiness of the buffers represents the dirtiness of
 * subsections of the page.  If the page has buffers, the page dirty bit is
 * merely a hint about the true dirty state.
 *
 * When a page is set dirty in its entirety, all its buffers are marked dirty
 * (if the page has buffers).
 *
 * When a buffer is marked dirty, its page is dirtied, but the page's other
 * buffers are not.
 *
 * Also.  When blockdev buffers are explicitly read with bread(), they
 * individually become uptodate.  But their backing page remains not
 * uptodate - even if all of its buffers are uptodate.  A subsequent
 * block_read_full_page() against that page will discover all the uptodate
 * buffers, will set the page uptodate and will perform no I/O.
 */

/**
 * mark_buffer_dirty - mark a buffer_head as needing writeout
 * @bh: the buffer_head to mark dirty
 *
 * mark_buffer_dirty() will set the dirty bit against the buffer, then set its
 * backing page dirty, then tag the page as dirty in its address_space's radix
 * tree and then attach the address_space's inode to its superblock's dirty
 * inode list.
 *
 * mark_buffer_dirty() is atomic.  It takes bh->b_page->mapping->private_lock,
 * mapping->tree_lock and mapping->host->i_lock.
 */
void mark_buffer_dirty(struct buffer_head *bh)
{
        WARN_ON_ONCE(!buffer_uptodate(bh));

        /*
         * Very *carefully* optimize the it-is-already-dirty case.
         *
         * Don't let the final "is it dirty" escape to before we
         * perhaps modified the buffer.
         */
        if (buffer_dirty(bh)) {
                smp_mb();
                if (buffer_dirty(bh))
                        return;
        }

        if (!test_set_buffer_dirty(bh)) {
                struct page *page = bh->b_page;
                if (!TestSetPageDirty(page)) {
                        struct address_space *mapping = page_mapping(page);
                        if (mapping)
                                __set_page_dirty(page, mapping, 0);
                }
        }
}
EXPORT_SYMBOL(mark_buffer_dirty);

/*
 * Decrement a buffer_head's reference count.  If all buffers against a page
 * have zero reference count, are clean and unlocked, and if the page is clean
 * and unlocked then try_to_free_buffers() may strip the buffers from the page
 * in preparation for freeing it (sometimes, rarely, buffers are removed from
 * a page but it ends up not being freed, and buffers may later be reattached).
 */
void __brelse(struct buffer_head * buf)
{
        if (atomic_read(&buf->b_count)) {
                put_bh(buf);
                return;
        }
        WARN(1, KERN_ERR "VFS: brelse: Trying to free free buffer\n");
}
EXPORT_SYMBOL(__brelse);

/*
 * bforget() is like brelse(), except it discards any
 * potentially dirty data.
 */
void __bforget(struct buffer_head *bh)
{
        clear_buffer_dirty(bh);
        if (bh->b_assoc_map) {
                struct address_space *buffer_mapping = bh->b_page->mapping;

                spin_lock(&buffer_mapping->private_lock);
                list_del_init(&bh->b_assoc_buffers);
                bh->b_assoc_map = NULL;
                spin_unlock(&buffer_mapping->private_lock);
        }
        __brelse(bh);
}
EXPORT_SYMBOL(__bforget);

static struct buffer_head *__bread_slow(struct buffer_head *bh)
{
        lock_buffer(bh);
        if (buffer_uptodate(bh)) {
                unlock_buffer(bh);
                return bh;
        } else {
                get_bh(bh);
                bh->b_end_io = end_buffer_read_sync;
                submit_bh(READ, bh);
                wait_on_buffer(bh);
                if (buffer_uptodate(bh))
                        return bh;
        }
        brelse(bh);
        return NULL;
}

/*
 * Per-cpu buffer LRU implementation.  To reduce the cost of __find_get_block().
 * The bhs[] array is sorted - newest buffer is at bhs[0].  Buffers have their
 * refcount elevated by one when they're in an LRU.  A buffer can only appear
 * once in a particular CPU's LRU.  A single buffer can be present in multiple
 * CPU's LRUs at the same time.
 *
 * This is a transparent caching front-end to sb_bread(), sb_getblk() and
 * sb_find_get_block().
 *
 * The LRUs themselves only need locking against invalidate_bh_lrus.  We use
 * a local interrupt disable for that.
 */

#define BH_LRU_SIZE     8

struct bh_lru {
        struct buffer_head *bhs[BH_LRU_SIZE];
};

static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }};

#ifdef CONFIG_SMP
#define bh_lru_lock()   local_irq_disable()
#define bh_lru_unlock() local_irq_enable()
#else
#define bh_lru_lock()   preempt_disable()
#define bh_lru_unlock() preempt_enable()
#endif

static inline void check_irqs_on(void)
{
#ifdef irqs_disabled
        BUG_ON(irqs_disabled());
#endif
}

/*
 * The LRU management algorithm is dopey-but-simple.  Sorry.
 */
static void bh_lru_install(struct buffer_head *bh)
{
        struct buffer_head *evictee = NULL;

        check_irqs_on();
        bh_lru_lock();
        if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
                struct buffer_head *bhs[BH_LRU_SIZE];
                int in;
                int out = 0;

                get_bh(bh);
                bhs[out++] = bh;
                for (in = 0; in < BH_LRU_SIZE; in++) {
                        struct buffer_head *bh2 =
                                __this_cpu_read(bh_lrus.bhs[in]);

                        if (bh2 == bh) {
                                __brelse(bh2);
                        } else {
                                if (out >= BH_LRU_SIZE) {
                                        BUG_ON(evictee != NULL);
                                        evictee = bh2;
                                } else {
                                        bhs[out++] = bh2;
                                }
                        }
                }
                while (out < BH_LRU_SIZE)
                        bhs[out++] = NULL;
                memcpy(__this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
        }
        bh_lru_unlock();

        if (evictee)
                __brelse(evictee);
}

/*
 * Look up the bh in this cpu's LRU.  If it's there, move it to the head.
 */
static struct buffer_head *
lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)
{
        struct buffer_head *ret = NULL;
        unsigned int i;

        check_irqs_on();
        bh_lru_lock();
        for (i = 0; i < BH_LRU_SIZE; i++) {
                struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);

                if (bh && bh->b_bdev == bdev &&
                                bh->b_blocknr == block && bh->b_size == size) {
                        if (i) {
                                while (i) {
                                        __this_cpu_write(bh_lrus.bhs[i],
                                                __this_cpu_read(bh_lrus.bhs[i - 1]));
                                        i--;
                                }
                                __this_cpu_write(bh_lrus.bhs[0], bh);
                        }
                        get_bh(bh);
                        ret = bh;
                        break;
                }
        }
        bh_lru_unlock();
        return ret;
}

/*
 * Perform a pagecache lookup for the matching buffer.  If it's there, refresh
 * it in the LRU and mark it as accessed.  If it is not present then return
 * NULL
 */
struct buffer_head *
__find_get_block(struct block_device *bdev, sector_t block, unsigned size)
{
        struct buffer_head *bh = lookup_bh_lru(bdev, block, size);

        if (bh == NULL) {
                bh = __find_get_block_slow(bdev, block);
                if (bh)
                        bh_lru_install(bh);
        }
        if (bh)
                touch_buffer(bh);
        return bh;
}
EXPORT_SYMBOL(__find_get_block);

/*
 * __getblk will locate (and, if necessary, create) the buffer_head
 * which corresponds to the passed block_device, block and size. The
 * returned buffer has its reference count incremented.
 *
 * __getblk() will lock up the machine if grow_dev_page's try_to_free_buffers()
 * attempt is failing.  FIXME, perhaps?
 */
struct buffer_head *
__getblk(struct block_device *bdev, sector_t block, unsigned size)
{
        struct buffer_head *bh = __find_get_block(bdev, block, size);

        might_sleep();
        if (bh == NULL)
                bh = __getblk_slow(bdev, block, size);
        return bh;
}
EXPORT_SYMBOL(__getblk);

/*
 * Do async read-ahead on a buffer..
 */
void __breadahead(struct block_device *bdev, sector_t block, unsigned size)
{
        struct buffer_head *bh = __getblk(bdev, block, size);
        if (likely(bh)) {
                ll_rw_block(READA, 1, &bh);
                brelse(bh);
        }
}
EXPORT_SYMBOL(__breadahead);

/**
 *  __bread() - reads a specified block and returns the bh
 *  @bdev: the block_device to read from
 *  @block: number of block
 *  @size: size (in bytes) to read
 * 
 *  Reads a specified block, and returns buffer head that contains it.
 *  It returns NULL if the block was unreadable.
 */
struct buffer_head *
__bread(struct block_device *bdev, sector_t block, unsigned size)
{
        struct buffer_head *bh = __getblk(bdev, block, size);

        if (likely(bh) && !buffer_uptodate(bh))
                bh = __bread_slow(bh);
        return bh;
}
EXPORT_SYMBOL(__bread);

/*
 * invalidate_bh_lrus() is called rarely - but not only at unmount.
 * This doesn't race because it runs in each cpu either in irq
 * or with preempt disabled.
 */
static void invalidate_bh_lru(void *arg)
{
        struct bh_lru *b = &get_cpu_var(bh_lrus);
        int i;

        for (i = 0; i < BH_LRU_SIZE; i++) {
                brelse(b->bhs[i]);
                b->bhs[i] = NULL;
        }
        put_cpu_var(bh_lrus);
}

static bool has_bh_in_lru(int cpu, void *dummy)
{
        struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
        int i;
        
        for (i = 0; i < BH_LRU_SIZE; i++) {
                if (b->bhs[i])
                        return 1;
        }

        return 0;
}

void invalidate_bh_lrus(void)
{
        on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1, GFP_KERNEL);
}
EXPORT_SYMBOL_GPL(invalidate_bh_lrus);

void set_bh_page(struct buffer_head *bh,
                struct page *page, unsigned long offset)
{
        bh->b_page = page;
        BUG_ON(offset >= PAGE_SIZE);
        if (PageHighMem(page))
                /*
                 * This catches illegal uses and preserves the offset:
                 */
                bh->b_data = (char *)(0 + offset);
        else
                bh->b_data = page_address(page) + offset;
}
EXPORT_SYMBOL(set_bh_page);

/*
 * Called when truncating a buffer on a page completely.
 */
static void discard_buffer(struct buffer_head * bh)
{
        lock_buffer(bh);
        clear_buffer_dirty(bh);
        bh->b_bdev = NULL;
        clear_buffer_mapped(bh);
        clear_buffer_req(bh);
        clear_buffer_new(bh);
        clear_buffer_delay(bh);
        clear_buffer_unwritten(bh);
        unlock_buffer(bh);
}

/**
 * block_invalidatepage - invalidate part or all of a buffer-backed page
 *
 * @page: the page which is affected
 * @offset: the index of the truncation point
 *
 * block_invalidatepage() is called when all or part of the page has become
 * invalidated by a truncate operation.
 *
 * block_invalidatepage() does not have to release all buffers, but it must
 * ensure that no dirty buffer is left outside @offset and that no I/O
 * is underway against any of the blocks which are outside the truncation
 * point.  Because the caller is about to free (and possibly reuse) those
 * blocks on-disk.
 */
void block_invalidatepage(struct page *page, unsigned long offset)
{
        struct buffer_head *head, *bh, *next;
        unsigned int curr_off = 0;

        BUG_ON(!PageLocked(page));
        if (!page_has_buffers(page))
                goto out;

        head = page_buffers(page);
        bh = head;
        do {
                unsigned int next_off = curr_off + bh->b_size;
                next = bh->b_this_page;

                /*
                 * is this block fully invalidated?
                 */
                if (offset <= curr_off)
                        discard_buffer(bh);
                curr_off = next_off;
                bh = next;
        } while (bh != head);

        /*
         * We release buffers only if the entire page is being invalidated.
         * The get_block cached value has been unconditionally invalidated,
         * so real IO is not possible anymore.
         */
        if (offset == 0)
                try_to_release_page(page, 0);
out:
        return;
}
EXPORT_SYMBOL(block_invalidatepage);

/*
 * We attach and possibly dirty the buffers atomically wrt
 * __set_page_dirty_buffers() via private_lock.  try_to_free_buffers
 * is already excluded via the page lock.
 */
void create_empty_buffers(struct page *page,
                        unsigned long blocksize, unsigned long b_state)
{
        struct buffer_head *bh, *head, *tail;

        head = alloc_page_buffers(page, blocksize, 1);
        bh = head;
        do {
                bh->b_state |= b_state;
                tail = bh;
                bh = bh->b_this_page;
        } while (bh);
        tail->b_this_page = head;

        spin_lock(&page->mapping->private_lock);
        if (PageUptodate(page) || PageDirty(page)) {
                bh = head;
                do {
                        if (PageDirty(page))
                                set_buffer_dirty(bh);
                        if (PageUptodate(page))
                                set_buffer_uptodate(bh);
                        bh = bh->b_this_page;
                } while (bh != head);
        }
        attach_page_buffers(page, head);
        spin_unlock(&page->mapping->private_lock);
}
EXPORT_SYMBOL(create_empty_buffers);

/*
 * We are taking a block for data and we don't want any output from any
 * buffer-cache aliases starting from return from that function and
 * until the moment when something will explicitly mark the buffer
 * dirty (hopefully that will not happen until we will free that block ;-)
 * We don't even need to mark it not-uptodate - nobody can expect
 * anything from a newly allocated buffer anyway. We used to used
 * unmap_buffer() for such invalidation, but that was wrong. We definitely
 * don't want to mark the alias unmapped, for example - it would confuse
 * anyone who might pick it with bread() afterwards...
 *
 * Also..  Note that bforget() doesn't lock the buffer.  So there can
 * be writeout I/O going on against recently-freed buffers.  We don't
 * wait on that I/O in bforget() - it's more efficient to wait on the I/O
 * only if we really need to.  That happens here.
 */
void unmap_underlying_metadata(struct block_device *bdev, sector_t block)
{
        struct buffer_head *old_bh;

        might_sleep();

        old_bh = __find_get_block_slow(bdev, block);
        if (old_bh) {
                clear_buffer_dirty(old_bh);
                wait_on_buffer(old_bh);
                clear_buffer_req(old_bh);
                __brelse(old_bh);
        }
}
EXPORT_SYMBOL(unmap_underlying_metadata);

/*
 * Size is a power-of-two in the range 512..PAGE_SIZE,
 * and the case we care about most is PAGE_SIZE.
 *
 * So this *could* possibly be written with those
 * constraints in mind (relevant mostly if some
 * architecture has a slow bit-scan instruction)
 */
static inline int block_size_bits(unsigned int blocksize)
{
        return ilog2(blocksize);
}

static struct buffer_head *create_page_buffers(struct page *page, struct inode *inode, unsigned int b_state)
{
        BUG_ON(!PageLocked(page));

        if (!page_has_buffers(page))
                create_empty_buffers(page, 1 << ACCESS_ONCE(inode->i_blkbits), b_state);
        return page_buffers(page);
}

/*
 * NOTE! All mapped/uptodate combinations are valid:
 *
 *      Mapped  Uptodate        Meaning
 *
 *      No      No              "unknown" - must do get_block()
 *      No      Yes             "hole" - zero-filled
 *      Yes     No              "allocated" - allocated on disk, not read in
 *      Yes     Yes             "valid" - allocated and up-to-date in memory.
 *
 * "Dirty" is valid only with the last case (mapped+uptodate).
 */

/*
 * While block_write_full_page is writing back the dirty buffers under
 * the page lock, whoever dirtied the buffers may decide to clean them
 * again at any time.  We handle that by only looking at the buffer
 * state inside lock_buffer().
 *
 * If block_write_full_page() is called for regular writeback
 * (wbc->sync_mode == WB_SYNC_NONE) then it will redirty a page which has a
 * locked buffer.   This only can happen if someone has written the buffer
 * directly, with submit_bh().  At the address_space level PageWriteback
 * prevents this contention from occurring.
 *
 * If block_write_full_page() is called with wbc->sync_mode ==
 * WB_SYNC_ALL, the writes are posted using WRITE_SYNC; this
 * causes the writes to be flagged as synchronous writes.
 */
static int __block_write_full_page(struct inode *inode, struct page *page,
                        get_block_t *get_block, struct writeback_control *wbc,
                        bh_end_io_t *handler)
{
        int err;
        sector_t block;
        sector_t last_block;
        struct buffer_head *bh, *head;
        unsigned int blocksize, bbits;
        int nr_underway = 0;
        int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
                        WRITE_SYNC : WRITE);

        head = create_page_buffers(page, inode,
                                        (1 << BH_Dirty)|(1 << BH_Uptodate));

        /*
         * Be very careful.  We have no exclusion from __set_page_dirty_buffers
         * here, and the (potentially unmapped) buffers may become dirty at
         * any time.  If a buffer becomes dirty here after we've inspected it
         * then we just miss that fact, and the page stays dirty.
         *
         * Buffers outside i_size may be dirtied by __set_page_dirty_buffers;
         * handle that here by just cleaning them.
         */

        bh = head;
        blocksize = bh->b_size;
        bbits = block_size_bits(blocksize);

        block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
        last_block = (i_size_read(inode) - 1) >> bbits;

        /*
         * Get all the dirty buffers mapped to disk addresses and
         * handle any aliases from the underlying blockdev's mapping.
         */
        do {
                if (block > last_block) {
                        /*
                         * mapped buffers outside i_size will occur, because
                         * this page can be outside i_size when there is a
                         * truncate in progress.
                         */
                        /*
                         * The buffer was zeroed by block_write_full_page()
                         */
                        clear_buffer_dirty(bh);
                        set_buffer_uptodate(bh);
                } else if ((!buffer_mapped(bh) || buffer_delay(bh)) &&
                           buffer_dirty(bh)) {
                        WARN_ON(bh->b_size != blocksize);
                        err = get_block(inode, block, bh, 1);
                        if (err)
                                goto recover;
                        clear_buffer_delay(bh);
                        if (buffer_new(bh)) {
                                /* blockdev mappings never come here */
                                clear_buffer_new(bh);
                                unmap_underlying_metadata(bh->b_bdev,
                                                        bh->b_blocknr);
                        }
                }
                bh = bh->b_this_page;
                block++;
        } while (bh != head);

        do {
                if (!buffer_mapped(bh))
                        continue;
                /*
                 * If it's a fully non-blocking write attempt and we cannot
                 * lock the buffer then redirty the page.  Note that this can
                 * potentially cause a busy-wait loop from writeback threads
                 * and kswapd activity, but those code paths have their own
                 * higher-level throttling.
                 */
                if (wbc->sync_mode != WB_SYNC_NONE) {
                        lock_buffer(bh);
                } else if (!trylock_buffer(bh)) {
                        redirty_page_for_writepage(wbc, page);
                        continue;
                }
                if (test_clear_buffer_dirty(bh)) {
                        mark_buffer_async_write_endio(bh, handler);
                } else {
                        unlock_buffer(bh);
                }
        } while ((bh = bh->b_this_page) != head);

        /*
         * The page and its buffers are protected by PageWriteback(), so we can
         * drop the bh refcounts early.
         */
        BUG_ON(PageWriteback(page));
        set_page_writeback(page);

        do {
                struct buffer_head *next = bh->b_this_page;
                if (buffer_async_write(bh)) {
                        submit_bh(write_op, bh);
                        nr_underway++;
                }
                bh = next;
        } while (bh != head);
        unlock_page(page);

        err = 0;
done:
        if (nr_underway == 0) {
                /*
                 * The page was marked dirty, but the buffers were
                 * clean.  Someone wrote them back by hand with
                 * ll_rw_block/submit_bh.  A rare case.
                 */
                end_page_writeback(page);

                /*
                 * The page and buffer_heads can be released at any time from
                 * here on.
                 */
        }
        return err;

recover:
        /*
         * ENOSPC, or some other error.  We may already have added some
         * blocks to the file, so we need to write these out to avoid
         * exposing stale data.
         * The page is currently locked and not marked for writeback
         */
        bh = head;
        /* Recovery: lock and submit the mapped buffers */
        do {
                if (buffer_mapped(bh) && buffer_dirty(bh) &&
                    !buffer_delay(bh)) {
                        lock_buffer(bh);
                        mark_buffer_async_write_endio(bh, handler);
                } else {
                        /*
                         * The buffer may have been set dirty during
                         * attachment to a dirty page.
                         */
                        clear_buffer_dirty(bh);
                }
        } while ((bh = bh->b_this_page) != head);
        SetPageError(page);
        BUG_ON(PageWriteback(page));
        mapping_set_error(page->mapping, err);
        set_page_writeback(page);
        do {
                struct buffer_head *next = bh->b_this_page;
                if (buffer_async_write(bh)) {
                        clear_buffer_dirty(bh);
                        submit_bh(write_op, bh);
                        nr_underway++;
                }
                bh = next;
        } while (bh != head);
        unlock_page(page);
        goto done;
}

/*
 * If a page has any new buffers, zero them out here, and mark them uptodate
 * and dirty so they'll be written out (in order to prevent uninitialised
 * block data from leaking). And clear the new bit.
 */
void page_zero_new_buffers(struct page *page, unsigned from, unsigned to)
{
        unsigned int block_start, block_end;
        struct buffer_head *head, *bh;

        BUG_ON(!PageLocked(page));
        if (!page_has_buffers(page))
                return;

        bh = head = page_buffers(page);
        block_start = 0;
        do {
                block_end = block_start + bh->b_size;

                if (buffer_new(bh)) {
                        if (block_end > from && block_start < to) {
                                if (!PageUptodate(page)) {
                                        unsigned start, size;

                                        start = max(from, block_start);
                                        size = min(to, block_end) - start;

                                        zero_user(page, start, size);
                                        set_buffer_uptodate(bh);
                                }

                                clear_buffer_new(bh);
                                mark_buffer_dirty(bh);
                        }
                }

                block_start = block_end;
                bh = bh->b_this_page;
        } while (bh != head);
}
EXPORT_SYMBOL(page_zero_new_buffers);

int __block_write_begin(struct page *page, loff_t pos, unsigned len,
                get_block_t *get_block)
{
        unsigned from = pos & (PAGE_CACHE_SIZE - 1);
        unsigned to = from + len;
        struct inode *inode = page->mapping->host;
        unsigned block_start, block_end;
        sector_t block;
        int err = 0;
        unsigned blocksize, bbits;
        struct buffer_head *bh, *head, *wait[2], **wait_bh=wait;

        BUG_ON(!PageLocked(page));
        BUG_ON(from > PAGE_CACHE_SIZE);
        BUG_ON(to > PAGE_CACHE_SIZE);
        BUG_ON(from > to);

        head = create_page_buffers(page, inode, 0);
        blocksize = head->b_size;
        bbits = block_size_bits(blocksize);

        block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);

        for(bh = head, block_start = 0; bh != head || !block_start;
            block++, block_start=block_end, bh = bh->b_this_page) {
                block_end = block_start + blocksize;
                if (block_end <= from || block_start >= to) {
                        if (PageUptodate(page)) {
                                if (!buffer_uptodate(bh))
                                        set_buffer_uptodate(bh);
                        }
                        continue;
                }
                if (buffer_new(bh))
                        clear_buffer_new(bh);
                if (!buffer_mapped(bh)) {
                        WARN_ON(bh->b_size != blocksize);
                        err = get_block(inode, block, bh, 1);
                        if (err)
                                break;
                        if (buffer_new(bh)) {
                                unmap_underlying_metadata(bh->b_bdev,
                                                        bh->b_blocknr);
                                if (PageUptodate(page)) {
                                        clear_buffer_new(bh);
                                        set_buffer_uptodate(bh);
                                        mark_buffer_dirty(bh);
                                        continue;
                                }
                                if (block_end > to || block_start < from)
                                        zero_user_segments(page,
                                                to, block_end,
                                                block_start, from);
                                continue;
                        }
                }
                if (PageUptodate(page)) {
                        if (!buffer_uptodate(bh))
                                set_buffer_uptodate(bh);
                        continue; 
                }
                if (!buffer_uptodate(bh) && !buffer_delay(bh) &&
                    !buffer_unwritten(bh) &&
                     (block_start < from || block_end > to)) {
                        ll_rw_block(READ, 1, &bh);
                        *wait_bh++=bh;
                }
        }
        /*
         * If we issued read requests - let them complete.
         */
        while(wait_bh > wait) {
                wait_on_buffer(*--wait_bh);
                if (!buffer_uptodate(*wait_bh))
                        err = -EIO;
        }
        if (unlikely(err))
                page_zero_new_buffers(page, from, to);
        return err;
}
EXPORT_SYMBOL(__block_write_begin);

static int __block_commit_write(struct inode *inode, struct page *page,
                unsigned from, unsigned to)
{
        unsigned block_start, block_end;
        int partial = 0;
        unsigned blocksize;
        struct buffer_head *bh, *head;

        bh = head = page_buffers(page);
        blocksize = bh->b_size;

        block_start = 0;
        do {
                block_end = block_start + blocksize;
                if (block_end <= from || block_start >= to) {
                        if (!buffer_uptodate(bh))
                                partial = 1;
                } else {
                        set_buffer_uptodate(bh);
                        mark_buffer_dirty(bh);
                }
                clear_buffer_new(bh);

                block_start = block_end;
                bh = bh->b_this_page;
        } while (bh != head);

        /*
         * If this is a partial write which happened to make all buffers
         * uptodate then we can optimize away a bogus readpage() for
         * the next read(). Here we 'discover' whether the page went
         * uptodate as a result of this (potentially partial) write.
         */
        if (!partial)
                SetPageUptodate(page);
        return 0;
}

/*
 * block_write_begin takes care of the basic task of block allocation and
 * bringing partial write blocks uptodate first.
 *
 * The filesystem needs to handle block truncation upon failure.
 */
int block_write_begin(struct address_space *mapping, loff_t pos, unsigned len,
                unsigned flags, struct page **pagep, get_block_t *get_block)
{
        pgoff_t index = pos >> PAGE_CACHE_SHIFT;
        struct page *page;
        int status;

        page = grab_cache_page_write_begin(mapping, index, flags);
        if (!page)
                return -ENOMEM;

        status = __block_write_begin(page, pos, len, get_block);
        if (unlikely(status)) {
                unlock_page(page);
                page_cache_release(page);
                page = NULL;
        }

        *pagep = page;
        return status;
}
EXPORT_SYMBOL(block_write_begin);

int block_write_end(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned copied,
                        struct page *page, void *fsdata)
{
        struct inode *inode = mapping->host;
        unsigned start;

        start = pos & (PAGE_CACHE_SIZE - 1);

        if (unlikely(copied < len)) {
                /*
                 * The buffers that were written will now be uptodate, so we
                 * don't have to worry about a readpage reading them and
                 * overwriting a partial write. However if we have encountered
                 * a short write and only partially written into a buffer, it
                 * will not be marked uptodate, so a readpage might come in and
                 * destroy our partial write.
                 *
                 * Do the simplest thing, and just treat any short write to a
                 * non uptodate page as a zero-length write, and force the
                 * caller to redo the whole thing.
                 */
                if (!PageUptodate(page))
                        copied = 0;

                page_zero_new_buffers(page, start+copied, start+len);
        }
        flush_dcache_page(page);

        /* This could be a short (even 0-length) commit */
        __block_commit_write(inode, page, start, start+copied);

        return copied;
}
EXPORT_SYMBOL(block_write_end);

int generic_write_end(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned copied,
                        struct page *page, void *fsdata)
{
        struct inode *inode = mapping->host;
        int i_size_changed = 0;

        copied = block_write_end(file, mapping, pos, len, copied, page, fsdata);

        /*
         * No need to use i_size_read() here, the i_size
         * cannot change under us because we hold i_mutex.
         *
         * But it's important to update i_size while still holding page lock:
         * page writeout could otherwise come in and zero beyond i_size.
         */
        if (pos+copied > inode->i_size) {
                i_size_write(inode, pos+copied);
                i_size_changed = 1;
        }

        unlock_page(page);
        page_cache_release(page);

        /*
         * Don't mark the inode dirty under page lock. First, it unnecessarily
         * makes the holding time of page lock longer. Second, it forces lock
         * ordering of page lock and transaction start for journaling
         * filesystems.
         */
        if (i_size_changed)
                mark_inode_dirty(inode);

        return copied;
}
EXPORT_SYMBOL(generic_write_end);

/*
 * block_is_partially_uptodate checks whether buffers within a page are
 * uptodate or not.
 *
 * Returns true if all buffers which correspond to a file portion
 * we want to read are uptodate.
 */
int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
                                        unsigned long from)
{
        unsigned block_start, block_end, blocksize;
        unsigned to;
        struct buffer_head *bh, *head;
        int ret = 1;

        if (!page_has_buffers(page))
                return 0;

        head = page_buffers(page);
        blocksize = head->b_size;
        to = min_t(unsigned, PAGE_CACHE_SIZE - from, desc->count);
        to = from + to;
        if (from < blocksize && to > PAGE_CACHE_SIZE - blocksize)
                return 0;

        bh = head;
        block_start = 0;
        do {
                block_end = block_start + blocksize;
                if (block_end > from && block_start < to) {
                        if (!buffer_uptodate(bh)) {
                                ret = 0;
                                break;
                        }
                        if (block_end >= to)
                                break;
                }
                block_start = block_end;
                bh = bh->b_this_page;
        } while (bh != head);

        return ret;
}
EXPORT_SYMBOL(block_is_partially_uptodate);

/*
 * Generic "read page" function for block devices that have the normal
 * get_block functionality. This is most of the block device filesystems.
 * Reads the page asynchronously --- the unlock_buffer() and
 * set/clear_buffer_uptodate() functions propagate buffer state into the
 * page struct once IO has completed.
 */
int block_read_full_page(struct page *page, get_block_t *get_block)
{
        struct inode *inode = page->mapping->host;
        sector_t iblock, lblock;
        struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
        unsigned int blocksize, bbits;
        int nr, i;
        int fully_mapped = 1;

        head = create_page_buffers(page, inode, 0);
        blocksize = head->b_size;
        bbits = block_size_bits(blocksize);

        iblock = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
        lblock = (i_size_read(inode)+blocksize-1) >> bbits;
        bh = head;
        nr = 0;
        i = 0;

        do {
                if (buffer_uptodate(bh))
                        continue;

                if (!buffer_mapped(bh)) {
                        int err = 0;

                        fully_mapped = 0;
                        if (iblock < lblock) {
                                WARN_ON(bh->b_size != blocksize);
                                err = get_block(inode, iblock, bh, 0);
                                if (err)
                                        SetPageError(page);
                        }
                        if (!buffer_mapped(bh)) {
                                zero_user(page, i * blocksize, blocksize);
                                if (!err)
                                        set_buffer_uptodate(bh);
                                continue;
                        }
                        /*
                         * get_block() might have updated the buffer
                         * synchronously
                         */
                        if (buffer_uptodate(bh))
                                continue;
                }
                arr[nr++] = bh;
        } while (i++, iblock++, (bh = bh->b_this_page) != head);

        if (fully_mapped)
                SetPageMappedToDisk(page);

        if (!nr) {
                /*
                 * All buffers are uptodate - we can set the page uptodate
                 * as well. But not if get_block() returned an error.
                 */
                if (!PageError(page))
                        SetPageUptodate(page);
                unlock_page(page);
                return 0;
        }

        /* Stage two: lock the buffers */
        for (i = 0; i < nr; i++) {
                bh = arr[i];
                lock_buffer(bh);
                mark_buffer_async_read(bh);
        }

        /*
         * Stage 3: start the IO.  Check for uptodateness
         * inside the buffer lock in case another process reading
         * the underlying blockdev brought it uptodate (the sct fix).
         */
        for (i = 0; i < nr; i++) {
                bh = arr[i];
                if (buffer_uptodate(bh))
                        end_buffer_async_read(bh, 1);
                else
                        submit_bh(READ, bh);
        }
        return 0;
}
EXPORT_SYMBOL(block_read_full_page);

/* utility function for filesystems that need to do work on expanding
 * truncates.  Uses filesystem pagecache writes to allow the filesystem to
 * deal with the hole.  
 */
int generic_cont_expand_simple(struct inode *inode, loff_t size)
{
        struct address_space *mapping = inode->i_mapping;
        struct page *page;
        void *fsdata;
        int err;

        err = inode_newsize_ok(inode, size);
        if (err)
                goto out;

        err = pagecache_write_begin(NULL, mapping, size, 0,
                                AOP_FLAG_UNINTERRUPTIBLE|AOP_FLAG_CONT_EXPAND,
                                &page, &fsdata);
        if (err)
                goto out;

        err = pagecache_write_end(NULL, mapping, size, 0, 0, page, fsdata);
        BUG_ON(err > 0);

out:
        return err;
}
EXPORT_SYMBOL(generic_cont_expand_simple);

static int cont_expand_zero(struct file *file, struct address_space *mapping,
                            loff_t pos, loff_t *bytes)
{
        struct inode *inode = mapping->host;
        unsigned blocksize = 1 << inode->i_blkbits;
        struct page *page;
        void *fsdata;
        pgoff_t index, curidx;
        loff_t curpos;
        unsigned zerofrom, offset, len;
        int err = 0;

        index = pos >> PAGE_CACHE_SHIFT;
        offset = pos & ~PAGE_CACHE_MASK;

        while (index > (curidx = (curpos = *bytes)>>PAGE_CACHE_SHIFT)) {
                zerofrom = curpos & ~PAGE_CACHE_MASK;
                if (zerofrom & (blocksize-1)) {
                        *bytes |= (blocksize-1);
                        (*bytes)++;
                }
                len = PAGE_CACHE_SIZE - zerofrom;

                err = pagecache_write_begin(file, mapping, curpos, len,
                                                AOP_FLAG_UNINTERRUPTIBLE,
                                                &page, &fsdata);
                if (err)
                        goto out;
                zero_user(page, zerofrom, len);
                err = pagecache_write_end(file, mapping, curpos, len, len,
                                                page, fsdata);
                if (err < 0)
                        goto out;
                BUG_ON(err != len);
                err = 0;

                balance_dirty_pages_ratelimited(mapping);
        }

        /* page covers the boundary, find the boundary offset */
        if (index == curidx) {
                zerofrom = curpos & ~PAGE_CACHE_MASK;
                /* if we will expand the thing last block will be filled */
                if (offset <= zerofrom) {
                        goto out;
                }
                if (zerofrom & (blocksize-1)) {
                        *bytes |= (blocksize-1);
                        (*bytes)++;
                }
                len = offset - zerofrom;

                err = pagecache_write_begin(file, mapping, curpos, len,
                                                AOP_FLAG_UNINTERRUPTIBLE,
                                                &page, &fsdata);
                if (err)
                        goto out;
                zero_user(page, zerofrom, len);
                err = pagecache_write_end(file, mapping, curpos, len, len,
                                                page, fsdata);
                if (err < 0)
                        goto out;
                BUG_ON(err != len);
                err = 0;
        }
out:
        return err;
}

/*
 * For moronic filesystems that do not allow holes in file.
 * We may have to extend the file.
 */
int cont_write_begin(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned flags,
                        struct page **pagep, void **fsdata,
                        get_block_t *get_block, loff_t *bytes)
{
        struct inode *inode = mapping->host;
        unsigned blocksize = 1 << inode->i_blkbits;
        unsigned zerofrom;
        int err;

        err = cont_expand_zero(file, mapping, pos, bytes);
        if (err)
                return err;

        zerofrom = *bytes & ~PAGE_CACHE_MASK;
        if (pos+len > *bytes && zerofrom & (blocksize-1)) {
                *bytes |= (blocksize-1);
                (*bytes)++;
        }

        return block_write_begin(mapping, pos, len, flags, pagep, get_block);
}
EXPORT_SYMBOL(cont_write_begin);

int block_commit_write(struct page *page, unsigned from, unsigned to)
{
        struct inode *inode = page->mapping->host;
        __block_commit_write(inode,page,from,to);
        return 0;
}
EXPORT_SYMBOL(block_commit_write);

/*
 * block_page_mkwrite() is not allowed to change the file size as it gets
 * called from a page fault handler when a page is first dirtied. Hence we must
 * be careful to check for EOF conditions here. We set the page up correctly
 * for a written page which means we get ENOSPC checking when writing into
 * holes and correct delalloc and unwritten extent mapping on filesystems that
 * support these features.
 *
 * We are not allowed to take the i_mutex here so we have to play games to
 * protect against truncate races as the page could now be beyond EOF.  Because
 * truncate writes the inode size before removing pages, once we have the
 * page lock we can determine safely if the page is beyond EOF. If it is not
 * beyond EOF, then the page is guaranteed safe against truncation until we
 * unlock the page.
 *
 * Direct callers of this function should protect against filesystem freezing
 * using sb_start_write() - sb_end_write() functions.
 */
int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
                         get_block_t get_block)
{
        struct page *page = vmf->page;
        struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
        unsigned long end;
        loff_t size;
        int ret;

        lock_page(page);
        size = i_size_read(inode);
        if ((page->mapping != inode->i_mapping) ||
            (page_offset(page) > size)) {
                /* We overload EFAULT to mean page got truncated */
                ret = -EFAULT;
                goto out_unlock;
        }

        /* page is wholly or partially inside EOF */
        if (((page->index + 1) << PAGE_CACHE_SHIFT) > size)
                end = size & ~PAGE_CACHE_MASK;
        else
                end = PAGE_CACHE_SIZE;

        ret = __block_write_begin(page, 0, end, get_block);
        if (!ret)
                ret = block_commit_write(page, 0, end);

        if (unlikely(ret < 0))
                goto out_unlock;
        set_page_dirty(page);
        wait_on_page_writeback(page);
        return 0;
out_unlock:
        unlock_page(page);
        return ret;
}
EXPORT_SYMBOL(__block_page_mkwrite);

int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
                   get_block_t get_block)
{
        int ret;
        struct super_block *sb = vma->vm_file->f_path.dentry->d_inode->i_sb;

        sb_start_pagefault(sb);

        /*
         * Update file times before taking page lock. We may end up failing the
         * fault so this update may be superfluous but who really cares...
         */
        file_update_time(vma->vm_file);

        ret = __block_page_mkwrite(vma, vmf, get_block);
        sb_end_pagefault(sb);
        return block_page_mkwrite_return(ret);
}
EXPORT_SYMBOL(block_page_mkwrite);

/*
 * nobh_write_begin()'s prereads are special: the buffer_heads are freed
 * immediately, while under the page lock.  So it needs a special end_io
 * handler which does not touch the bh after unlocking it.
 */
static void end_buffer_read_nobh(struct buffer_head *bh, int uptodate)
{
        __end_buffer_read_notouch(bh, uptodate);
}

/*
 * Attach the singly-linked list of buffers created by nobh_write_begin, to
 * the page (converting it to circular linked list and taking care of page
 * dirty races).
 */
static void attach_nobh_buffers(struct page *page, struct buffer_head *head)
{
        struct buffer_head *bh;

        BUG_ON(!PageLocked(page));

        spin_lock(&page->mapping->private_lock);
        bh = head;
        do {
                if (PageDirty(page))
                        set_buffer_dirty(bh);
                if (!bh->b_this_page)
                        bh->b_this_page = head;
                bh = bh->b_this_page;
        } while (bh != head);
        attach_page_buffers(page, head);
        spin_unlock(&page->mapping->private_lock);
}

/*
 * On entry, the page is fully not uptodate.
 * On exit the page is fully uptodate in the areas outside (from,to)
 * The filesystem needs to handle block truncation upon failure.
 */
int nobh_write_begin(struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned flags,
                        struct page **pagep, void **fsdata,
                        get_block_t *get_block)
{
        struct inode *inode = mapping->host;
        const unsigned blkbits = inode->i_blkbits;
        const unsigned blocksize = 1 << blkbits;
        struct buffer_head *head, *bh;
        struct page *page;
        pgoff_t index;
        unsigned from, to;
        unsigned block_in_page;
        unsigned block_start, block_end;
        sector_t block_in_file;
        int nr_reads = 0;
        int ret = 0;
        int is_mapped_to_disk = 1;

        index = pos >> PAGE_CACHE_SHIFT;
        from = pos & (PAGE_CACHE_SIZE - 1);
        to = from + len;

        page = grab_cache_page_write_begin(mapping, index, flags);
        if (!page)
                return -ENOMEM;
        *pagep = page;
        *fsdata = NULL;

        if (page_has_buffers(page)) {
                ret = __block_write_begin(page, pos, len, get_block);
                if (unlikely(ret))
                        goto out_release;
                return ret;
        }

        if (PageMappedToDisk(page))
                return 0;

        /*
         * Allocate buffers so that we can keep track of state, and potentially
         * attach them to the page if an error occurs. In the common case of
         * no error, they will just be freed again without ever being attached
         * to the page (which is all OK, because we're under the page lock).
         *
         * Be careful: the buffer linked list is a NULL terminated one, rather
         * than the circular one we're used to.
         */
        head = alloc_page_buffers(page, blocksize, 0);
        if (!head) {
                ret = -ENOMEM;
                goto out_release;
        }

        block_in_file = (sector_t)page->index << (PAGE_CACHE_SHIFT - blkbits);

        /*
         * We loop across all blocks in the page, whether or not they are
         * part of the affected region.  This is so we can discover if the
         * page is fully mapped-to-disk.
         */
        for (block_start = 0, block_in_page = 0, bh = head;
                  block_start < PAGE_CACHE_SIZE;
                  block_in_page++, block_start += blocksize, bh = bh->b_this_page) {
                int create;

                block_end = block_start + blocksize;
                bh->b_state = 0;
                create = 1;
                if (block_start >= to)
                        create = 0;
                ret = get_block(inode, block_in_file + block_in_page,
                                        bh, create);
                if (ret)
                        goto failed;
                if (!buffer_mapped(bh))
                        is_mapped_to_disk = 0;
                if (buffer_new(bh))
                        unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
                if (PageUptodate(page)) {
                        set_buffer_uptodate(bh);
                        continue;
                }
                if (buffer_new(bh) || !buffer_mapped(bh)) {
                        zero_user_segments(page, block_start, from,
                                                        to, block_end);
                        continue;
                }
                if (buffer_uptodate(bh))
                        continue;       /* reiserfs does this */
                if (block_start < from || block_end > to) {
                        lock_buffer(bh);
                        bh->b_end_io = end_buffer_read_nobh;
                        submit_bh(READ, bh);
                        nr_reads++;
                }
        }

        if (nr_reads) {
                /*
                 * The page is locked, so these buffers are protected from
                 * any VM or truncate activity.  Hence we don't need to care
                 * for the buffer_head refcounts.
                 */
                for (bh = head; bh; bh = bh->b_this_page) {
                        wait_on_buffer(bh);
                        if (!buffer_uptodate(bh))
                                ret = -EIO;
                }
                if (ret)
                        goto failed;
        }

        if (is_mapped_to_disk)
                SetPageMappedToDisk(page);

        *fsdata = head; /* to be released by nobh_write_end */

        return 0;

failed:
        BUG_ON(!ret);
        /*
         * Error recovery is a bit difficult. We need to zero out blocks that
         * were newly allocated, and dirty them to ensure they get written out.
         * Buffers need to be attached to the page at this point, otherwise
         * the handling of potential IO errors during writeout would be hard
         * (could try doing synchronous writeout, but what if that fails too?)
         */
        attach_nobh_buffers(page, head);
        page_zero_new_buffers(page, from, to);

out_release:
        unlock_page(page);
        page_cache_release(page);
        *pagep = NULL;

        return ret;
}
EXPORT_SYMBOL(nobh_write_begin);

int nobh_write_end(struct file *file, struct address_space *mapping,
                        loff_t pos, unsigned len, unsigned copied,
                        struct page *page, void *fsdata)
{
        struct inode *inode = page->mapping->host;
        struct buffer_head *head = fsdata;
        struct buffer_head *bh;
        BUG_ON(fsdata != NULL && page_has_buffers(page));

        if (unlikely(copied < len) && head)
                attach_nobh_buffers(page, head);
        if (page_has_buffers(page))
                return generic_write_end(file, mapping, pos, len,
                                        copied, page, fsdata);

        SetPageUptodate(page);
        set_page_dirty(page);
        if (pos+copied > inode->i_size) {
                i_size_write(inode, pos+copied);
                mark_inode_dirty(inode);
        }

        unlock_page(page);
        page_cache_release(page);

        while (head) {
                bh = head;
                head = head->b_this_page;
                free_buffer_head(bh);
        }

        return copied;
}
EXPORT_SYMBOL(nobh_write_end);

/*
 * nobh_writepage() - based on block_full_write_page() except
 * that it tries to operate without attaching bufferheads to
 * the page.
 */
int nobh_writepage(struct page *page, get_block_t *get_block,
                        struct writeback_control *wbc)
{
        struct inode * const inode = page->mapping->host;
        loff_t i_size = i_size_read(inode);
        const pgoff_t end_index = i_size >> PAGE_CACHE_SHIFT;
        unsigned offset;
        int ret;

        /* Is the page fully inside i_size? */
        if (page->index < end_index)
                goto out;

        /* Is the page fully outside i_size? (truncate in progress) */
        offset = i_size & (PAGE_CACHE_SIZE-1);
        if (page->index >= end_index+1 || !offset) {
                /*
                 * The page may have dirty, unmapped buffers.  For example,
                 * they may have been added in ext3_writepage().  Make them
                 * freeable here, so the page does not leak.
                 */
#if 0
                /* Not really sure about this  - do we need this ? */
                if (page->mapping->a_ops->invalidatepage)
                        page->mapping->a_ops->invalidatepage(page, offset);
#endif
                unlock_page(page);
                return 0; /* don't care */
        }

        /*
         * The page straddles i_size.  It must be zeroed out on each and every
         * writepage invocation because it may be mmapped.  "A file is mapped
         * in multiples of the page size.  For a file that is not a multiple of
         * the  page size, the remaining memory is zeroed when mapped, and
         * writes to that region are not written out to the file."
         */
        zero_user_segment(page, offset, PAGE_CACHE_SIZE);
out:
        ret = mpage_writepage(page, get_block, wbc);
        if (ret == -EAGAIN)
                ret = __block_write_full_page(inode, page, get_block, wbc,
                                              end_buffer_async_write);
        return ret;
}
EXPORT_SYMBOL(nobh_writepage);

int nobh_truncate_page(struct address_space *mapping,
                        loff_t from, get_block_t *get_block)
{
        pgoff_t index = from >> PAGE_CACHE_SHIFT;
        unsigned offset = from & (PAGE_CACHE_SIZE-1);
        unsigned blocksize;
        sector_t iblock;
        unsigned length, pos;
        struct inode *inode = mapping->host;
        struct page *page;
        struct buffer_head map_bh;
        int err;

        blocksize = 1 << inode->i_blkbits;
        length = offset & (blocksize - 1);

        /* Block boundary? Nothing to do */
        if (!length)
                return 0;

        length = blocksize - length;
        iblock = (sector_t)index << (PAGE_CACHE_SHIFT - inode->i_blkbits);

        page = grab_cache_page(mapping, index);
        err = -ENOMEM;
        if (!page)
                goto out;

        if (page_has_buffers(page)) {
has_buffers:
                unlock_page(page);
                page_cache_release(page);
                return block_truncate_page(mapping, from, get_block);
        }

        /* Find the buffer that contains "offset" */
        pos = blocksize;
        while (offset >= pos) {
                iblock++;
                pos += blocksize;
        }

        map_bh.b_size = blocksize;
        map_bh.b_state = 0;
        err = get_block(inode, iblock, &map_bh, 0);
        if (err)
                goto unlock;
        /* unmapped? It's a hole - nothing to do */
        if (!buffer_mapped(&map_bh))
                goto unlock;

        /* Ok, it's mapped. Make sure it's up-to-date */
        if (!PageUptodate(page)) {
                err = mapping->a_ops->readpage(NULL, page);
                if (err) {
                        page_cache_release(page);
                        goto out;
                }
                lock_page(page);
                if (!PageUptodate(page)) {
                        err = -EIO;
                        goto unlock;
                }
                if (page_has_buffers(page))
                        goto has_buffers;
        }
        zero_user(page, offset, length);
        set_page_dirty(page);
        err = 0;

unlock:
        unlock_page(page);
        page_cache_release(page);
out:
        return err;
}
EXPORT_SYMBOL(nobh_truncate_page);

int block_truncate_page(struct address_space *mapping,
                        loff_t from, get_block_t *get_block)
{
        pgoff_t index = from >> PAGE_CACHE_SHIFT;
        unsigned offset = from & (PAGE_CACHE_SIZE-1);
        unsigned blocksize;
        sector_t iblock;
        unsigned length, pos;
        struct inode *inode = mapping->host;
        struct page *page;
        struct buffer_head *bh;
        int err;

        blocksize = 1 << inode->i_blkbits;
        length = offset & (blocksize - 1);

        /* Block boundary? Nothing to do */
        if (!length)
                return 0;

        length = blocksize - length;
        iblock = (sector_t)index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
        
        page = grab_cache_page(mapping, index);
        err = -ENOMEM;
        if (!page)
                goto out;

        if (!page_has_buffers(page))
                create_empty_buffers(page, blocksize, 0);

        /* Find the buffer that contains "offset" */
        bh = page_buffers(page);
        pos = blocksize;
        while (offset >= pos) {
                bh = bh->b_this_page;
                iblock++;
                pos += blocksize;
        }

        err = 0;
        if (!buffer_mapped(bh)) {
                WARN_ON(bh->b_size != blocksize);
                err = get_block(inode, iblock, bh, 0);
                if (err)
                        goto unlock;
                /* unmapped? It's a hole - nothing to do */
                if (!buffer_mapped(bh))
                        goto unlock;
        }

        /* Ok, it's mapped. Make sure it's up-to-date */
        if (PageUptodate(page))
                set_buffer_uptodate(bh);

        if (!buffer_uptodate(bh) && !buffer_delay(bh) && !buffer_unwritten(bh)) {
                err = -EIO;
                ll_rw_block(READ, 1, &bh);
                wait_on_buffer(bh);
                /* Uhhuh. Read error. Complain and punt. */
                if (!buffer_uptodate(bh))
                        goto unlock;
        }

        zero_user(page, offset, length);
        mark_buffer_dirty(bh);
        err = 0;

unlock:
        unlock_page(page);
        page_cache_release(page);
out:
        return err;
}
EXPORT_SYMBOL(block_truncate_page);

/*
 * The generic ->writepage function for buffer-backed address_spaces
 * this form passes in the end_io handler used to finish the IO.
 */
int block_write_full_page_endio(struct page *page, get_block_t *get_block,
                        struct writeback_control *wbc, bh_end_io_t *handler)
{
        struct inode * const inode = page->mapping->host;
        loff_t i_size = i_size_read(inode);
        const pgoff_t end_index = i_size >> PAGE_CACHE_SHIFT;
        unsigned offset;

        /* Is the page fully inside i_size? */
        if (page->index < end_index)
                return __block_write_full_page(inode, page, get_block, wbc,
                                               handler);

        /* Is the page fully outside i_size? (truncate in progress) */
        offset = i_size & (PAGE_CACHE_SIZE-1);
        if (page->index >= end_index+1 || !offset) {
                /*
                 * The page may have dirty, unmapped buffers.  For example,
                 * they may have been added in ext3_writepage().  Make them
                 * freeable here, so the page does not leak.
                 */
                do_invalidatepage(page, 0);
                unlock_page(page);
                return 0; /* don't care */
        }

        /*
         * The page straddles i_size.  It must be zeroed out on each and every
         * writepage invocation because it may be mmapped.  "A file is mapped
         * in multiples of the page size.  For a file that is not a multiple of
         * the  page size, the remaining memory is zeroed when mapped, and
         * writes to that region are not written out to the file."
         */
        zero_user_segment(page, offset, PAGE_CACHE_SIZE);
        return __block_write_full_page(inode, page, get_block, wbc, handler);
}
EXPORT_SYMBOL(block_write_full_page_endio);

/*
 * The generic ->writepage function for buffer-backed address_spaces
 */
int block_write_full_page(struct page *page, get_block_t *get_block,
                        struct writeback_control *wbc)
{
        return block_write_full_page_endio(page, get_block, wbc,
                                           end_buffer_async_write);
}
EXPORT_SYMBOL(block_write_full_page);

sector_t generic_block_bmap(struct address_space *mapping, sector_t block,
                            get_block_t *get_block)
{
        struct buffer_head tmp;
        struct inode *inode = mapping->host;
        tmp.b_state = 0;
        tmp.b_blocknr = 0;
        tmp.b_size = 1 << inode->i_blkbits;
        get_block(inode, block, &tmp, 0);
        return tmp.b_blocknr;
}
EXPORT_SYMBOL(generic_block_bmap);

static void end_bio_bh_io_sync(struct bio *bio, int err)
{
        struct buffer_head *bh = bio->bi_private;

        if (err == -EOPNOTSUPP) {
                set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
        }

        if (unlikely (test_bit(BIO_QUIET,&bio->bi_flags)))
                set_bit(BH_Quiet, &bh->b_state);

        bh->b_end_io(bh, test_bit(BIO_UPTODATE, &bio->bi_flags));
        bio_put(bio);
}

/*
 * This allows us to do IO even on the odd last sectors
 * of a device, even if the bh block size is some multiple
 * of the physical sector size.
 *
 * We'll just truncate the bio to the size of the device,
 * and clear the end of the buffer head manually.
 *
 * Truly out-of-range accesses will turn into actual IO
 * errors, this only handles the "we need to be able to
 * do IO at the final sector" case.
 */
static void guard_bh_eod(int rw, struct bio *bio, struct buffer_head *bh)
{
        sector_t maxsector;
        unsigned bytes;

        maxsector = i_size_read(bio->bi_bdev->bd_inode) >> 9;
        if (!maxsector)
                return;

        /*
         * If the *whole* IO is past the end of the device,
         * let it through, and the IO layer will turn it into
         * an EIO.
         */
        if (unlikely(bio->bi_sector >= maxsector))
                return;

        maxsector -= bio->bi_sector;
        bytes = bio->bi_size;
        if (likely((bytes >> 9) <= maxsector))
                return;

        /* Uhhuh. We've got a bh that straddles the device size! */
        bytes = maxsector << 9;

        /* Truncate the bio.. */
        bio->bi_size = bytes;
        bio->bi_io_vec[0].bv_len = bytes;

        /* ..and clear the end of the buffer for reads */
        if ((rw & RW_MASK) == READ) {
                void *kaddr = kmap_atomic(bh->b_page);
                memset(kaddr + bh_offset(bh) + bytes, 0, bh->b_size - bytes);
                kunmap_atomic(kaddr);
        }
}

int submit_bh(int rw, struct buffer_head * bh)
{
        struct bio *bio;
        int ret = 0;

        BUG_ON(!buffer_locked(bh));
        BUG_ON(!buffer_mapped(bh));
        BUG_ON(!bh->b_end_io);
        BUG_ON(buffer_delay(bh));
        BUG_ON(buffer_unwritten(bh));

        /*
         * Only clear out a write error when rewriting
         */
        if (test_set_buffer_req(bh) && (rw & WRITE))
                clear_buffer_write_io_error(bh);

        /*
         * from here on down, it's all bio -- do the initial mapping,
         * submit_bio -> generic_make_request may further map this bio around
         */
        bio = bio_alloc(GFP_NOIO, 1);

        bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
        bio->bi_bdev = bh->b_bdev;
        bio->bi_io_vec[0].bv_page = bh->b_page;
        bio->bi_io_vec[0].bv_len = bh->b_size;
        bio->bi_io_vec[0].bv_offset = bh_offset(bh);

        bio->bi_vcnt = 1;
        bio->bi_idx = 0;
        bio->bi_size = bh->b_size;

        bio->bi_end_io = end_bio_bh_io_sync;
        bio->bi_private = bh;

        /* Take care of bh's that straddle the end of the device */
        guard_bh_eod(rw, bio, bh);

        bio_get(bio);
        submit_bio(rw, bio);

        if (bio_flagged(bio, BIO_EOPNOTSUPP))
                ret = -EOPNOTSUPP;

        bio_put(bio);
        return ret;
}
EXPORT_SYMBOL(submit_bh);

/**
 * ll_rw_block: low-level access to block devices (DEPRECATED)
 * @rw: whether to %READ or %WRITE or maybe %READA (readahead)
 * @nr: number of &struct buffer_heads in the array
 * @bhs: array of pointers to &struct buffer_head
 *
 * ll_rw_block() takes an array of pointers to &struct buffer_heads, and
 * requests an I/O operation on them, either a %READ or a %WRITE.  The third
 * %READA option is described in the documentation for generic_make_request()
 * which ll_rw_block() calls.
 *
 * This function drops any buffer that it cannot get a lock on (with the
 * BH_Lock state bit), any buffer that appears to be clean when doing a write
 * request, and any buffer that appears to be up-to-date when doing read
 * request.  Further it marks as clean buffers that are processed for
 * writing (the buffer cache won't assume that they are actually clean
 * until the buffer gets unlocked).
 *
 * ll_rw_block sets b_end_io to simple completion handler that marks
 * the buffer up-to-date (if approriate), unlocks the buffer and wakes
 * any waiters. 
 *
 * All of the buffers must be for the same device, and must also be a
 * multiple of the current approved size for the device.
 */
void ll_rw_block(int rw, int nr, struct buffer_head *bhs[])
{
        int i;

        for (i = 0; i < nr; i++) {
                struct buffer_head *bh = bhs[i];

                if (!trylock_buffer(bh))
                        continue;
                if (rw == WRITE) {
                        if (test_clear_buffer_dirty(bh)) {
                                bh->b_end_io = end_buffer_write_sync;
                                get_bh(bh);
                                submit_bh(WRITE, bh);
                                continue;
                        }
                } else {
                        if (!buffer_uptodate(bh)) {
                                bh->b_end_io = end_buffer_read_sync;
                                get_bh(bh);
                                submit_bh(rw, bh);
                                continue;
                        }
                }
                unlock_buffer(bh);
        }
}
EXPORT_SYMBOL(ll_rw_block);

void write_dirty_buffer(struct buffer_head *bh, int rw)
{
        lock_buffer(bh);
        if (!test_clear_buffer_dirty(bh)) {
                unlock_buffer(bh);
                return;
        }
        bh->b_end_io = end_buffer_write_sync;
        get_bh(bh);
        submit_bh(rw, bh);
}
EXPORT_SYMBOL(write_dirty_buffer);

/*
 * For a data-integrity writeout, we need to wait upon any in-progress I/O
 * and then start new I/O and then wait upon it.  The caller must have a ref on
 * the buffer_head.
 */
int __sync_dirty_buffer(struct buffer_head *bh, int rw)
{
        int ret = 0;

        WARN_ON(atomic_read(&bh->b_count) < 1);
        lock_buffer(bh);
        if (test_clear_buffer_dirty(bh)) {
                get_bh(bh);
                bh->b_end_io = end_buffer_write_sync;
                ret = submit_bh(rw, bh);
                wait_on_buffer(bh);
                if (!ret && !buffer_uptodate(bh))
                        ret = -EIO;
        } else {
                unlock_buffer(bh);
        }
        return ret;
}
EXPORT_SYMBOL(__sync_dirty_buffer);

int sync_dirty_buffer(struct buffer_head *bh)
{
        return __sync_dirty_buffer(bh, WRITE_SYNC);
}
EXPORT_SYMBOL(sync_dirty_buffer);

/*
 * try_to_free_buffers() checks if all the buffers on this particular page
 * are unused, and releases them if so.
 *
 * Exclusion against try_to_free_buffers may be obtained by either
 * locking the page or by holding its mapping's private_lock.
 *
 * If the page is dirty but all the buffers are clean then we need to
 * be sure to mark the page clean as well.  This is because the page
 * may be against a block device, and a later reattachment of buffers
 * to a dirty page will set *all* buffers dirty.  Which would corrupt
 * filesystem data on the same device.
 *
 * The same applies to regular filesystem pages: if all the buffers are
 * clean then we set the page clean and proceed.  To do that, we require
 * total exclusion from __set_page_dirty_buffers().  That is obtained with
 * private_lock.
 *
 * try_to_free_buffers() is non-blocking.
 */
static inline int buffer_busy(struct buffer_head *bh)
{
        return atomic_read(&bh->b_count) |
                (bh->b_state & ((1 << BH_Dirty) | (1 << BH_Lock)));
}

static int
drop_buffers(struct page *page, struct buffer_head **buffers_to_free)
{
        struct buffer_head *head = page_buffers(page);
        struct buffer_head *bh;

        bh = head;
        do {
                if (buffer_write_io_error(bh) && page->mapping)
                        set_bit(AS_EIO, &page->mapping->flags);
                if (buffer_busy(bh))
                        goto failed;
                bh = bh->b_this_page;
        } while (bh != head);

        do {
                struct buffer_head *next = bh->b_this_page;

                if (bh->b_assoc_map)
                        __remove_assoc_queue(bh);
                bh = next;
        } while (bh != head);
        *buffers_to_free = head;
        __clear_page_buffers(page);
        return 1;
failed:
        return 0;
}

int try_to_free_buffers(struct page *page)
{
        struct address_space * const mapping = page->mapping;
        struct buffer_head *buffers_to_free = NULL;
        int ret = 0;

        BUG_ON(!PageLocked(page));
        if (PageWriteback(page))
                return 0;

        if (mapping == NULL) {          /* can this still happen? */
                ret = drop_buffers(page, &buffers_to_free);
                goto out;
        }

        spin_lock(&mapping->private_lock);
        ret = drop_buffers(page, &buffers_to_free);

        /*
         * If the filesystem writes its buffers by hand (eg ext3)
         * then we can have clean buffers against a dirty page.  We
         * clean the page here; otherwise the VM will never notice
         * that the filesystem did any IO at all.
         *
         * Also, during truncate, discard_buffer will have marked all
         * the page's buffers clean.  We discover that here and clean
         * the page also.
         *
         * private_lock must be held over this entire operation in order
         * to synchronise against __set_page_dirty_buffers and prevent the
         * dirty bit from being lost.
         */
        if (ret)
                cancel_dirty_page(page, PAGE_CACHE_SIZE);
        spin_unlock(&mapping->private_lock);
out:
        if (buffers_to_free) {
                struct buffer_head *bh = buffers_to_free;

                do {
                        struct buffer_head *next = bh->b_this_page;
                        free_buffer_head(bh);
                        bh = next;
                } while (bh != buffers_to_free);
        }
        return ret;
}
EXPORT_SYMBOL(try_to_free_buffers);

/*
 * There are no bdflush tunables left.  But distributions are
 * still running obsolete flush daemons, so we terminate them here.
 *
 * Use of bdflush() is deprecated and will be removed in a future kernel.
 * The `flush-X' kernel threads fully replace bdflush daemons and this call.
 */
SYSCALL_DEFINE2(bdflush, int, func, long, data)
{
        static int msg_count;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        if (msg_count < 5) {
                msg_count++;
                printk(KERN_INFO
                        "warning: process `%s' used the obsolete bdflush"
                        " system call\n", current->comm);
                printk(KERN_INFO "Fix your initscripts?\n");
        }

        if (func == 1)
                do_exit(0);
        return 0;
}

/*
 * Buffer-head allocation
 */
static struct kmem_cache *bh_cachep __read_mostly;

/*
 * Once the number of bh's in the machine exceeds this level, we start
 * stripping them in writeback.
 */
static int max_buffer_heads;

int buffer_heads_over_limit;

struct bh_accounting {
        int nr;                 /* Number of live bh's */
        int ratelimit;          /* Limit cacheline bouncing */
};

static DEFINE_PER_CPU(struct bh_accounting, bh_accounting) = {0, 0};

static void recalc_bh_state(void)
{
        int i;
        int tot = 0;

        if (__this_cpu_inc_return(bh_accounting.ratelimit) - 1 < 4096)
                return;
        __this_cpu_write(bh_accounting.ratelimit, 0);
        for_each_online_cpu(i)
                tot += per_cpu(bh_accounting, i).nr;
        buffer_heads_over_limit = (tot > max_buffer_heads);
}

struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
{
        struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
        if (ret) {
                INIT_LIST_HEAD(&ret->b_assoc_buffers);
                preempt_disable();
                __this_cpu_inc(bh_accounting.nr);
                recalc_bh_state();
                preempt_enable();
        }
        return ret;
}
EXPORT_SYMBOL(alloc_buffer_head);

void free_buffer_head(struct buffer_head *bh)
{
        BUG_ON(!list_empty(&bh->b_assoc_buffers));
        kmem_cache_free(bh_cachep, bh);
        preempt_disable();
        __this_cpu_dec(bh_accounting.nr);
        recalc_bh_state();
        preempt_enable();
}
EXPORT_SYMBOL(free_buffer_head);

static void buffer_exit_cpu(int cpu)
{
        int i;
        struct bh_lru *b = &per_cpu(bh_lrus, cpu);

        for (i = 0; i < BH_LRU_SIZE; i++) {
                brelse(b->bhs[i]);
                b->bhs[i] = NULL;
        }
        this_cpu_add(bh_accounting.nr, per_cpu(bh_accounting, cpu).nr);
        per_cpu(bh_accounting, cpu).nr = 0;
}

static int buffer_cpu_notify(struct notifier_block *self,
                              unsigned long action, void *hcpu)
{
        if (action == CPU_DEAD || action == CPU_DEAD_FROZEN)
                buffer_exit_cpu((unsigned long)hcpu);
        return NOTIFY_OK;
}

/**
 * bh_uptodate_or_lock - Test whether the buffer is uptodate
 * @bh: struct buffer_head
 *
 * Return true if the buffer is up-to-date and false,
 * with the buffer locked, if not.
 */
int bh_uptodate_or_lock(struct buffer_head *bh)
{
        if (!buffer_uptodate(bh)) {
                lock_buffer(bh);
                if (!buffer_uptodate(bh))
                        return 0;
                unlock_buffer(bh);
        }
        return 1;
}
EXPORT_SYMBOL(bh_uptodate_or_lock);

/**
 * bh_submit_read - Submit a locked buffer for reading
 * @bh: struct buffer_head
 *
 * Returns zero on success and -EIO on error.
 */
int bh_submit_read(struct buffer_head *bh)
{
        BUG_ON(!buffer_locked(bh));

        if (buffer_uptodate(bh)) {
                unlock_buffer(bh);
                return 0;
        }

        get_bh(bh);
        bh->b_end_io = end_buffer_read_sync;
        submit_bh(READ, bh);
        wait_on_buffer(bh);
        if (buffer_uptodate(bh))
                return 0;
        return -EIO;
}
EXPORT_SYMBOL(bh_submit_read);

void __init buffer_init(void)
{
        int nrpages;

        bh_cachep = kmem_cache_create("buffer_head",
                        sizeof(struct buffer_head), 0,
                                (SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|
                                SLAB_MEM_SPREAD),
                                NULL);

        /*
         * Limit the bh occupancy to 10% of ZONE_NORMAL
         */
        nrpages = (nr_free_buffer_pages() * 10) / 100;
        max_buffer_heads = nrpages * (PAGE_SIZE / sizeof(struct buffer_head));
        hotcpu_notifier(buffer_cpu_notify, 0);
}