Import 2.3.13pre6

[davej-history.git] / mm / filemap.c

/*
 *      linux/mm/filemap.c
 *
 * Copyright (C) 1994-1999  Linus Torvalds
 */

/*
 * This file handles the generic file mmap semantics used by
 * most "normal" filesystems (but you don't /have/ to use this:
 * the NFS filesystem used to do this differently, for example)
 */
#include <linux/malloc.h>
#include <linux/shm.h>
#include <linux/mman.h>
#include <linux/locks.h>
#include <linux/pagemap.h>
#include <linux/swap.h>
#include <linux/smp_lock.h>
#include <linux/blkdev.h>
#include <linux/file.h>
#include <linux/swapctl.h>
#include <linux/slab.h>
#include <linux/init.h>

#include <asm/pgtable.h>
#include <asm/uaccess.h>

/*
 * Shared mappings implemented 30.11.1994. It's not fully working yet,
 * though.
 *
 * Shared mappings now work. 15.8.1995  Bruno.
 *
 * finished 'unifying' the page and buffer cache and SMP-threaded the
 * page-cache, 21.05.1999, Ingo Molnar <mingo@redhat.com>
 */

atomic_t page_cache_size = ATOMIC_INIT(0);
unsigned int page_hash_bits;
struct page **page_hash_table;

spinlock_t pagecache_lock = SPIN_LOCK_UNLOCKED;


void __add_page_to_hash_queue(struct page * page, struct page **p)
{
        atomic_inc(&page_cache_size);
        if((page->next_hash = *p) != NULL)
                (*p)->pprev_hash = &page->next_hash;
        *p = page;
        page->pprev_hash = p;
        if (page->buffers)
                PAGE_BUG(page);
}

static void remove_page_from_hash_queue(struct page * page)
{
        if(page->pprev_hash) {
                if(page->next_hash)
                        page->next_hash->pprev_hash = page->pprev_hash;
                *page->pprev_hash = page->next_hash;
                page->pprev_hash = NULL;
        }
        atomic_dec(&page_cache_size);
}

static void remove_page_from_inode_queue(struct page * page)
{
        struct inode * inode = page->inode;
        struct page *prev, *next;

        inode->i_nrpages--;
        next = page->next;
        prev = page->prev;
        if (inode->i_pages == page)
                inode->i_pages = next;
        if (next)
                next->prev = prev;
        if (prev)
                prev->next = next;
        page->next = NULL;
        page->prev = NULL;
}

/*
 * Remove a page from the page cache and free it. Caller has to make
 * sure the page is locked and that nobody else uses it - or that usage
 * is safe.
 */
void remove_inode_page(struct page *page)
{
        if (!PageLocked(page))
                PAGE_BUG(page);

        spin_lock(&pagecache_lock);
        remove_page_from_inode_queue(page);
        remove_page_from_hash_queue(page);
        page->inode = NULL;
        spin_unlock(&pagecache_lock);
}

void invalidate_inode_pages(struct inode * inode)
{
        struct page ** p;
        struct page * page;

repeat:
        spin_lock(&pagecache_lock);
        p = &inode->i_pages;
        while ((page = *p) != NULL) {
                get_page(page);
                if (TryLockPage(page)) {
                        spin_unlock(&pagecache_lock);
                        wait_on_page(page);
                        page_cache_release(page);
                        goto repeat;
                }
                if (page_count(page) != 2)
                        printk("hm, busy page invalidated? (not necesserily a bug)\n");

                remove_page_from_inode_queue(page);
                remove_page_from_hash_queue(page);
                page->inode = NULL;
                UnlockPage(page);
                page_cache_release(page);
                page_cache_release(page);

        }
        spin_unlock(&pagecache_lock);
}
/*
 * Truncate the page cache at a set offset, removing the pages
 * that are beyond that offset (and zeroing out partial pages).
 */
void truncate_inode_pages(struct inode * inode, unsigned long start)
{
        struct page ** p;
        struct page * page;
        int partial = 0;

repeat:
        spin_lock(&pagecache_lock);
        p = &inode->i_pages;
        while ((page = *p) != NULL) {
                unsigned long offset = page->offset;

                /* page wholly truncated - free it */
                if (offset >= start) {
                        get_page(page);
                        spin_unlock(&pagecache_lock);

                        lock_page(page);

                        if (inode->i_op->flushpage)
                                inode->i_op->flushpage(inode, page, 0);

                        /*
                         * We remove the page from the page cache
                         * _after_ we have destroyed all buffer-cache
                         * references to it. Otherwise some other process
                         * might think this inode page is not in the
                         * page cache and creates a buffer-cache alias
                         * to it causing all sorts of fun problems ...
                         */
                        remove_inode_page(page);

                        UnlockPage(page);
                        page_cache_release(page);
                        page_cache_release(page);

                        /*
                         * We have done things without the pagecache lock,
                         * so we'll have to repeat the scan.
                         * It's not possible to deadlock here because
                         * we are guaranteed to make progress. (ie. we have
                         * just removed a page)
                         */
                        goto repeat;
                }
                p = &page->next;
                /*
                 * there is only one partial page possible.
                 */
                if (partial)
                        continue;

                offset = start - offset;
                /* partial truncate, clear end of page */
                if (offset < PAGE_CACHE_SIZE) {
                        unsigned long address;
                        get_page(page);
                        spin_unlock(&pagecache_lock);

                        lock_page(page);
                        partial = 1;

                        address = page_address(page);
                        memset((void *) (offset + address), 0, PAGE_CACHE_SIZE - offset);
                        flush_page_to_ram(address);

                        if (inode->i_op->flushpage)
                                inode->i_op->flushpage(inode, page, offset);
                        /*
                         * we have dropped the spinlock so we have to
                         * restart.
                         */
                        UnlockPage(page);
                        page_cache_release(page);
                        goto repeat;
                }
        }
        spin_unlock(&pagecache_lock);
}

extern atomic_t too_many_dirty_buffers;

int shrink_mmap(int priority, int gfp_mask)
{
        static unsigned long clock = 0;
        unsigned long limit = num_physpages << 1;
        struct page * page;
        int count, users;

        count = limit >> priority;

        page = mem_map + clock;
        do {
                int referenced;

                /* This works even in the presence of PageSkip because
                 * the first two entries at the beginning of a hole will
                 * be marked, not just the first.
                 */
                page++;
                clock++;
                if (clock >= max_mapnr) {
                        clock = 0;
                        page = mem_map;
                }
                if (PageSkip(page)) {
                        /* next_hash is overloaded for PageSkip */
                        page = page->next_hash;
                        clock = page - mem_map;
                }
                
                referenced = test_and_clear_bit(PG_referenced, &page->flags);

                if ((gfp_mask & __GFP_DMA) && !PageDMA(page))
                        continue;

                count--;

                /*
                 * Some common cases that we just short-circuit without
                 * getting the locks - we need to re-check this once we
                 * have the lock, but that's fine.
                 */
                users = page_count(page);
                if (!users)
                        continue;
                if (!page->buffers) {
                        if (!page->inode)
                                continue;
                        if (users > 1)
                                continue;
                }

                /*
                 * ok, now the page looks interesting. Re-check things
                 * and keep the lock.
                 */
                spin_lock(&pagecache_lock);
                if (!page->inode && !page->buffers) {
                        spin_unlock(&pagecache_lock);
                        continue;
                }
                if (!page_count(page)) {
                        spin_unlock(&pagecache_lock);
                        BUG();
                        continue;
                }
                get_page(page);
                if (TryLockPage(page)) {
                        spin_unlock(&pagecache_lock);
                        goto put_continue;
                }

                /*
                 * we keep pagecache_lock locked and unlock it in
                 * each branch, so that the page->inode case doesnt
                 * have to re-grab it. Here comes the 'real' logic
                 * to free memory:
                 */

                /* Is it a buffer page? */
                if (page->buffers) {
                        spin_unlock(&pagecache_lock);
                        if (!try_to_free_buffers(page))
                                goto unlock_continue;
                        /* page was locked, inode can't go away under us */
                        if (!page->inode)
                        {
                                atomic_sub(PAGE_CACHE_SIZE, &buffermem);
                                goto made_progress;
                        }
                        spin_lock(&pagecache_lock);
                }

                /*
                 * We can't free pages unless there's just one user
                 * (count == 2 because we added one ourselves above).
                 */
                if (page_count(page) != 2)
                        goto spin_unlock_continue;

                /*
                 * Is it a page swap page? If so, we want to
                 * drop it if it is no longer used, even if it
                 * were to be marked referenced..
                 */
                if (PageSwapCache(page)) {
                        spin_unlock(&pagecache_lock);
                        if (referenced && swap_count(page->offset) != 2)
                                goto unlock_continue;
                        __delete_from_swap_cache(page);
                        page_cache_release(page);
                        goto made_progress;
                }       

                /* is it a page-cache page? */
                if (!referenced && page->inode && !pgcache_under_min()) {
                        remove_page_from_inode_queue(page);
                        remove_page_from_hash_queue(page);
                        page->inode = NULL;
                        spin_unlock(&pagecache_lock);

                        page_cache_release(page);
                        goto made_progress;
                }
spin_unlock_continue:
                spin_unlock(&pagecache_lock);
unlock_continue:
                UnlockPage(page);
put_continue:
                put_page(page);
        } while (count > 0);
        return 0;
made_progress:
        UnlockPage(page);
        put_page(page);
        return 1;
}

static inline struct page * __find_page_nolock(struct inode * inode, unsigned long offset, struct page *page)
{
        goto inside;

        for (;;) {
                page = page->next_hash;
inside:
                if (!page)
                        goto not_found;
                if (page->inode != inode)
                        continue;
                if (page->offset == offset)
                        break;
        }
        set_bit(PG_referenced, &page->flags);
not_found:
        return page;
}

/*
 * By the time this is called, the page is locked and
 * we don't have to worry about any races any more.
 *
 * Start the IO..
 */
static int writeout_one_page(struct page *page)
{
        struct buffer_head *bh, *head = page->buffers;

        bh = head;
        do {
                if (buffer_locked(bh) || !buffer_dirty(bh) || !buffer_uptodate(bh))
                        continue;

                bh->b_flushtime = 0;
                ll_rw_block(WRITE, 1, &bh);     
        } while ((bh = bh->b_this_page) != head);
        return 0;
}

static int waitfor_one_page(struct page *page)
{
        int error = 0;
        struct buffer_head *bh, *head = page->buffers;

        bh = head;
        do {
                wait_on_buffer(bh);
                if (buffer_req(bh) && !buffer_uptodate(bh))
                        error = -EIO;
        } while ((bh = bh->b_this_page) != head);
        return error;
}

static int do_buffer_fdatasync(struct inode *inode, unsigned long start, unsigned long end, int (*fn)(struct page *))
{
        struct page *next;
        int retval = 0;

        start &= PAGE_MASK;

        spin_lock(&pagecache_lock);
        next = inode->i_pages;
        while (next) {
                struct page *page = next;
                next = page->next;
                if (!page->buffers)
                        continue;
                if (page->offset >= end)
                        continue;
                if (page->offset < start)
                        continue;

                get_page(page);
                spin_unlock(&pagecache_lock);
                lock_page(page);

                /* The buffers could have been free'd while we waited for the page lock */
                if (page->buffers)
                        retval |= fn(page);

                UnlockPage(page);
                spin_lock(&pagecache_lock);
                next = page->next;
                page_cache_release(page);
        }
        spin_unlock(&pagecache_lock);

        return retval;
}

/*
 * Two-stage data sync: first start the IO, then go back and
 * collect the information..
 */
int generic_buffer_fdatasync(struct inode *inode, unsigned long start, unsigned long end)
{
        int retval;

        retval = do_buffer_fdatasync(inode, start, end, writeout_one_page);
        retval |= do_buffer_fdatasync(inode, start, end, waitfor_one_page);
        return retval;
}

/*
 * This adds a page to the page cache, starting out as locked,
 * owned by us, referenced, but not uptodate and with no errors.
 */
static inline void __add_to_page_cache(struct page * page,
        struct inode * inode, unsigned long offset,
        struct page **hash)
{
        unsigned long flags;

        flags = page->flags & ~((1 << PG_uptodate) | (1 << PG_error));
        page->flags = flags |  ((1 << PG_locked) | (1 << PG_referenced));
        page->owner = current;  /* REMOVEME */
        get_page(page);
        page->offset = offset;
        add_page_to_inode_queue(inode, page);
        __add_page_to_hash_queue(page, hash);
}

void add_to_page_cache(struct page * page, struct inode * inode, unsigned long offset)
{
        spin_lock(&pagecache_lock);
        __add_to_page_cache(page, inode, offset, page_hash(inode, offset));
        spin_unlock(&pagecache_lock);
}

int add_to_page_cache_unique(struct page * page,
        struct inode * inode, unsigned long offset,
        struct page **hash)
{
        int err;
        struct page *alias;

        spin_lock(&pagecache_lock);
        alias = __find_page_nolock(inode, offset, *hash);

        err = 1;
        if (!alias) {
                __add_to_page_cache(page,inode,offset,hash);
                err = 0;
        }

        spin_unlock(&pagecache_lock);
        return err;
}

/*
 * Try to read ahead in the file. "page_cache" is a potentially free page
 * that we could use for the cache (if it is 0 we can try to create one,
 * this is all overlapped with the IO on the previous page finishing anyway)
 */
static unsigned long try_to_read_ahead(struct file * file,
                                unsigned long offset, unsigned long page_cache)
{
        struct inode *inode = file->f_dentry->d_inode;
        struct page * page;
        struct page ** hash;

        offset &= PAGE_CACHE_MASK;
        switch (page_cache) {
        case 0:
                page_cache = page_cache_alloc();
                if (!page_cache)
                        break;
        default:
                if (offset >= inode->i_size)
                        break;
                hash = page_hash(inode, offset);
                page = page_cache_entry(page_cache);
                if (!add_to_page_cache_unique(page, inode, offset, hash)) {
                        /*
                         * We do not have to check the return value here
                         * because it's a readahead.
                         */
                        inode->i_op->readpage(file, page);
                        page_cache = 0;
                        page_cache_release(page);
                }
        }
        return page_cache;
}

/* 
 * Wait for a page to get unlocked.
 *
 * This must be called with the caller "holding" the page,
 * ie with increased "page->count" so that the page won't
 * go away during the wait..
 */
void ___wait_on_page(struct page *page)
{
        struct task_struct *tsk = current;
        DECLARE_WAITQUEUE(wait, tsk);

        add_wait_queue(&page->wait, &wait);
        do {
                tsk->state = TASK_UNINTERRUPTIBLE;
                run_task_queue(&tq_disk);
                if (!PageLocked(page))
                        break;
                schedule();
        } while (PageLocked(page));
        tsk->state = TASK_RUNNING;
        remove_wait_queue(&page->wait, &wait);
}

/*
 * Get an exclusive lock on the page..
 */
void lock_page(struct page *page)
{
        if (TryLockPage(page)) {
                struct task_struct *tsk = current;
                DECLARE_WAITQUEUE(wait, current);

                run_task_queue(&tq_disk);
                add_wait_queue(&page->wait, &wait);
                tsk->state = TASK_UNINTERRUPTIBLE;

                while (TryLockPage(page)) {
                        run_task_queue(&tq_disk);
                        schedule();
                        tsk->state = TASK_UNINTERRUPTIBLE;
                }

                remove_wait_queue(&page->wait, &wait);
                tsk->state = TASK_RUNNING;
        }
}


/*
 * a rather lightweight function, finding and getting a reference to a
 * hashed page atomically, waiting for it if it's locked.
 */
struct page * __find_get_page (struct inode * inode,
                                unsigned long offset, struct page **hash)
{
        struct page *page;

        /*
         * We scan the hash list read-only. Addition to and removal from
         * the hash-list needs a held write-lock.
         */
repeat:
        spin_lock(&pagecache_lock);
        page = __find_page_nolock(inode, offset, *hash);
        if (page)
                get_page(page);
        spin_unlock(&pagecache_lock);

        /* Found the page, sleep if locked. */
        if (page && PageLocked(page)) {
                struct task_struct *tsk = current;
                DECLARE_WAITQUEUE(wait, tsk);

                add_wait_queue(&page->wait, &wait);
                tsk->state = TASK_UNINTERRUPTIBLE;

                run_task_queue(&tq_disk);
                if (PageLocked(page))
                        schedule();
                tsk->state = TASK_RUNNING;
                remove_wait_queue(&page->wait, &wait);

                /*
                 * The page might have been unhashed meanwhile. It's
                 * not freed though because we hold a reference to it.
                 * If this is the case then it will be freed _here_,
                 * and we recheck the hash anyway.
                 */
                page_cache_release(page);
                goto repeat;
        }
        /*
         * It's not locked so we can return the page and we hold
         * a reference to it.
         */
        return page;
}

/*
 * Get the lock to a page atomically.
 */
struct page * __find_lock_page (struct inode * inode,
                                unsigned long offset, struct page **hash)
{
        struct page *page;

        /*
         * We scan the hash list read-only. Addition to and removal from
         * the hash-list needs a held write-lock.
         */
repeat:
        spin_lock(&pagecache_lock);
        page = __find_page_nolock(inode, offset, *hash);
        if (page)
                get_page(page);
        spin_unlock(&pagecache_lock);

        /* Found the page, sleep if locked. */
        if (page && TryLockPage(page)) {
                struct task_struct *tsk = current;
                DECLARE_WAITQUEUE(wait, tsk);

                add_wait_queue(&page->wait, &wait);
                tsk->state = TASK_UNINTERRUPTIBLE;

                run_task_queue(&tq_disk);
                if (PageLocked(page))
                        schedule();
                tsk->state = TASK_RUNNING;
                remove_wait_queue(&page->wait, &wait);

                /*
                 * The page might have been unhashed meanwhile. It's
                 * not freed though because we hold a reference to it.
                 * If this is the case then it will be freed _here_,
                 * and we recheck the hash anyway.
                 */
                page_cache_release(page);
                goto repeat;
        }
        /*
         * It's not locked so we can return the page and we hold
         * a reference to it.
         */
        return page;
}

#if 0
#define PROFILE_READAHEAD
#define DEBUG_READAHEAD
#endif

/*
 * Read-ahead profiling information
 * --------------------------------
 * Every PROFILE_MAXREADCOUNT, the following information is written 
 * to the syslog:
 *   Percentage of asynchronous read-ahead.
 *   Average of read-ahead fields context value.
 * If DEBUG_READAHEAD is defined, a snapshot of these fields is written 
 * to the syslog.
 */

#ifdef PROFILE_READAHEAD

#define PROFILE_MAXREADCOUNT 1000

static unsigned long total_reada;
static unsigned long total_async;
static unsigned long total_ramax;
static unsigned long total_ralen;
static unsigned long total_rawin;

static void profile_readahead(int async, struct file *filp)
{
        unsigned long flags;

        ++total_reada;
        if (async)
                ++total_async;

        total_ramax     += filp->f_ramax;
        total_ralen     += filp->f_ralen;
        total_rawin     += filp->f_rawin;

        if (total_reada > PROFILE_MAXREADCOUNT) {
                save_flags(flags);
                cli();
                if (!(total_reada > PROFILE_MAXREADCOUNT)) {
                        restore_flags(flags);
                        return;
                }

                printk("Readahead average:  max=%ld, len=%ld, win=%ld, async=%ld%%\n",
                        total_ramax/total_reada,
                        total_ralen/total_reada,
                        total_rawin/total_reada,
                        (total_async*100)/total_reada);
#ifdef DEBUG_READAHEAD
                printk("Readahead snapshot: max=%ld, len=%ld, win=%ld, raend=%ld\n",
                        filp->f_ramax, filp->f_ralen, filp->f_rawin, filp->f_raend);
#endif

                total_reada     = 0;
                total_async     = 0;
                total_ramax     = 0;
                total_ralen     = 0;
                total_rawin     = 0;

                restore_flags(flags);
        }
}
#endif  /* defined PROFILE_READAHEAD */

/*
 * Read-ahead context:
 * -------------------
 * The read ahead context fields of the "struct file" are the following:
 * - f_raend : position of the first byte after the last page we tried to
 *             read ahead.
 * - f_ramax : current read-ahead maximum size.
 * - f_ralen : length of the current IO read block we tried to read-ahead.
 * - f_rawin : length of the current read-ahead window.
 *              if last read-ahead was synchronous then
 *                      f_rawin = f_ralen
 *              otherwise (was asynchronous)
 *                      f_rawin = previous value of f_ralen + f_ralen
 *
 * Read-ahead limits:
 * ------------------
 * MIN_READAHEAD   : minimum read-ahead size when read-ahead.
 * MAX_READAHEAD   : maximum read-ahead size when read-ahead.
 *
 * Synchronous read-ahead benefits:
 * --------------------------------
 * Using reasonable IO xfer length from peripheral devices increase system 
 * performances.
 * Reasonable means, in this context, not too large but not too small.
 * The actual maximum value is:
 *      MAX_READAHEAD + PAGE_CACHE_SIZE = 76k is CONFIG_READA_SMALL is undefined
 *      and 32K if defined (4K page size assumed).
 *
 * Asynchronous read-ahead benefits:
 * ---------------------------------
 * Overlapping next read request and user process execution increase system 
 * performance.
 *
 * Read-ahead risks:
 * -----------------
 * We have to guess which further data are needed by the user process.
 * If these data are often not really needed, it's bad for system 
 * performances.
 * However, we know that files are often accessed sequentially by 
 * application programs and it seems that it is possible to have some good 
 * strategy in that guessing.
 * We only try to read-ahead files that seems to be read sequentially.
 *
 * Asynchronous read-ahead risks:
 * ------------------------------
 * In order to maximize overlapping, we must start some asynchronous read 
 * request from the device, as soon as possible.
 * We must be very careful about:
 * - The number of effective pending IO read requests.
 *   ONE seems to be the only reasonable value.
 * - The total memory pool usage for the file access stream.
 *   This maximum memory usage is implicitly 2 IO read chunks:
 *   2*(MAX_READAHEAD + PAGE_CACHE_SIZE) = 156K if CONFIG_READA_SMALL is undefined,
 *   64k if defined (4K page size assumed).
 */

static inline int get_max_readahead(struct inode * inode)
{
        if (!inode->i_dev || !max_readahead[MAJOR(inode->i_dev)])
                return MAX_READAHEAD;
        return max_readahead[MAJOR(inode->i_dev)][MINOR(inode->i_dev)];
}

static inline unsigned long generic_file_readahead(int reada_ok,
        struct file * filp, struct inode * inode,
        unsigned long ppos, struct page * page, unsigned long page_cache)
{
        unsigned long max_ahead, ahead;
        unsigned long raend;
        int max_readahead = get_max_readahead(inode);

        raend = filp->f_raend & PAGE_CACHE_MASK;
        max_ahead = 0;

/*
 * The current page is locked.
 * If the current position is inside the previous read IO request, do not
 * try to reread previously read ahead pages.
 * Otherwise decide or not to read ahead some pages synchronously.
 * If we are not going to read ahead, set the read ahead context for this 
 * page only.
 */
        if (PageLocked(page)) {
                if (!filp->f_ralen || ppos >= raend || ppos + filp->f_ralen < raend) {
                        raend = ppos;
                        if (raend < inode->i_size)
                                max_ahead = filp->f_ramax;
                        filp->f_rawin = 0;
                        filp->f_ralen = PAGE_CACHE_SIZE;
                        if (!max_ahead) {
                                filp->f_raend  = ppos + filp->f_ralen;
                                filp->f_rawin += filp->f_ralen;
                        }
                }
        }
/*
 * The current page is not locked.
 * If we were reading ahead and,
 * if the current max read ahead size is not zero and,
 * if the current position is inside the last read-ahead IO request,
 *   it is the moment to try to read ahead asynchronously.
 * We will later force unplug device in order to force asynchronous read IO.
 */
        else if (reada_ok && filp->f_ramax && raend >= PAGE_CACHE_SIZE &&
                 ppos <= raend && ppos + filp->f_ralen >= raend) {
/*
 * Add ONE page to max_ahead in order to try to have about the same IO max size
 * as synchronous read-ahead (MAX_READAHEAD + 1)*PAGE_CACHE_SIZE.
 * Compute the position of the last page we have tried to read in order to 
 * begin to read ahead just at the next page.
 */
                raend -= PAGE_CACHE_SIZE;
                if (raend < inode->i_size)
                        max_ahead = filp->f_ramax + PAGE_CACHE_SIZE;

                if (max_ahead) {
                        filp->f_rawin = filp->f_ralen;
                        filp->f_ralen = 0;
                        reada_ok      = 2;
                }
        }
/*
 * Try to read ahead pages.
 * We hope that ll_rw_blk() plug/unplug, coalescence, requests sort and the
 * scheduler, will work enough for us to avoid too bad actuals IO requests.
 */
        ahead = 0;
        while (ahead < max_ahead) {
                ahead += PAGE_CACHE_SIZE;
                page_cache = try_to_read_ahead(filp, raend + ahead,
                                                page_cache);
        }
/*
 * If we tried to read ahead some pages,
 * If we tried to read ahead asynchronously,
 *   Try to force unplug of the device in order to start an asynchronous
 *   read IO request.
 * Update the read-ahead context.
 * Store the length of the current read-ahead window.
 * Double the current max read ahead size.
 *   That heuristic avoid to do some large IO for files that are not really
 *   accessed sequentially.
 */
        if (ahead) {
                if (reada_ok == 2) {
                        run_task_queue(&tq_disk);
                }

                filp->f_ralen += ahead;
                filp->f_rawin += filp->f_ralen;
                filp->f_raend = raend + ahead + PAGE_CACHE_SIZE;

                filp->f_ramax += filp->f_ramax;

                if (filp->f_ramax > max_readahead)
                        filp->f_ramax = max_readahead;

#ifdef PROFILE_READAHEAD
                profile_readahead((reada_ok == 2), filp);
#endif
        }

        return page_cache;
}

/*
 * "descriptor" for what we're up to with a read.
 * This allows us to use the same read code yet
 * have multiple different users of the data that
 * we read from a file.
 *
 * The simplest case just copies the data to user
 * mode.
 */
typedef struct {
        size_t written;
        size_t count;
        char * buf;
        int error;
} read_descriptor_t;

typedef int (*read_actor_t)(read_descriptor_t *, const char *, unsigned long);

/*
 * This is a generic file read routine, and uses the
 * inode->i_op->readpage() function for the actual low-level
 * stuff.
 *
 * This is really ugly. But the goto's actually try to clarify some
 * of the logic when it comes to error handling etc.
 */
static void do_generic_file_read(struct file * filp, loff_t *ppos, read_descriptor_t * desc, read_actor_t actor)
{
        struct dentry *dentry = filp->f_dentry;
        struct inode *inode = dentry->d_inode;
        size_t pos, pgpos, page_cache;
        int reada_ok;
        int error;
        int max_readahead = get_max_readahead(inode);

        page_cache = 0;

        pos = *ppos;
        pgpos = pos & PAGE_CACHE_MASK;
/*
 * If the current position is outside the previous read-ahead window, 
 * we reset the current read-ahead context and set read ahead max to zero
 * (will be set to just needed value later),
 * otherwise, we assume that the file accesses are sequential enough to
 * continue read-ahead.
 */
        if (pgpos > filp->f_raend || pgpos + filp->f_rawin < filp->f_raend) {
                reada_ok = 0;
                filp->f_raend = 0;
                filp->f_ralen = 0;
                filp->f_ramax = 0;
                filp->f_rawin = 0;
        } else {
                reada_ok = 1;
        }
/*
 * Adjust the current value of read-ahead max.
 * If the read operation stay in the first half page, force no readahead.
 * Otherwise try to increase read ahead max just enough to do the read request.
 * Then, at least MIN_READAHEAD if read ahead is ok,
 * and at most MAX_READAHEAD in all cases.
 */
        if (pos + desc->count <= (PAGE_CACHE_SIZE >> 1)) {
                filp->f_ramax = 0;
        } else {
                unsigned long needed;

                needed = ((pos + desc->count) & PAGE_CACHE_MASK) - pgpos;

                if (filp->f_ramax < needed)
                        filp->f_ramax = needed;

                if (reada_ok && filp->f_ramax < MIN_READAHEAD)
                                filp->f_ramax = MIN_READAHEAD;
                if (filp->f_ramax > max_readahead)
                        filp->f_ramax = max_readahead;
        }

        for (;;) {
                struct page *page, **hash;

                if (pos >= inode->i_size)
                        break;

                /*
                 * Try to find the data in the page cache..
                 */
                hash = page_hash(inode, pos & PAGE_CACHE_MASK);

                spin_lock(&pagecache_lock);
                page = __find_page_nolock(inode, pos & PAGE_CACHE_MASK, *hash);
                if (!page)
                        goto no_cached_page;
found_page:
                get_page(page);
                spin_unlock(&pagecache_lock);

                if (!Page_Uptodate(page))
                        goto page_not_up_to_date;
page_ok:
        /*
         * Ok, we have the page, and it's up-to-date, so
         * now we can copy it to user space...
         */
        {
                unsigned long offset, nr;

                offset = pos & ~PAGE_CACHE_MASK;
                nr = PAGE_CACHE_SIZE - offset;
                if (nr > inode->i_size - pos)
                        nr = inode->i_size - pos;

                /*
                 * The actor routine returns how many bytes were actually used..
                 * NOTE! This may not be the same as how much of a user buffer
                 * we filled up (we may be padding etc), so we can only update
                 * "pos" here (the actor routine has to update the user buffer
                 * pointers and the remaining count).
                 */
                nr = actor(desc, (const char *) (page_address(page) + offset), nr);
                pos += nr;
                page_cache_release(page);
                if (nr && desc->count)
                        continue;
                break;
        }

/*
 * Ok, the page was not immediately readable, so let's try to read ahead while we're at it..
 */
page_not_up_to_date:
                page_cache = generic_file_readahead(reada_ok, filp, inode, pos & PAGE_CACHE_MASK, page, page_cache);

                if (Page_Uptodate(page))
                        goto page_ok;

                /* Get exclusive access to the page ... */
                lock_page(page);
                if (Page_Uptodate(page)) {
                        UnlockPage(page);
                        goto page_ok;
                }

readpage:
                /* ... and start the actual read. The read will unlock the page. */
                error = inode->i_op->readpage(filp, page);

                if (!error) {
                        if (Page_Uptodate(page))
                                goto page_ok;

                        /* Again, try some read-ahead while waiting for the page to finish.. */
                        page_cache = generic_file_readahead(reada_ok, filp, inode, pos & PAGE_CACHE_MASK, page, page_cache);
                        wait_on_page(page);
                        if (Page_Uptodate(page))
                                goto page_ok;
                        error = -EIO;
                }

                /* UHHUH! A synchronous read error occurred. Report it */
                desc->error = error;
                page_cache_release(page);
                break;

no_cached_page:
                /*
                 * Ok, it wasn't cached, so we need to create a new
                 * page..
                 *
                 * We get here with the page cache lock held.
                 */
                if (!page_cache) {
                        spin_unlock(&pagecache_lock);
                        page_cache = page_cache_alloc();
                        if (!page_cache) {
                                desc->error = -ENOMEM;
                                break;
                        }

                        /*
                         * Somebody may have added the page while we
                         * dropped the page cache lock. Check for that.
                         */
                        spin_lock(&pagecache_lock);
                        page = __find_page_nolock(inode, pos & PAGE_CACHE_MASK, *hash);
                        if (page)
                                goto found_page;
                }

                /*
                 * Ok, add the new page to the hash-queues...
                 */
                page = page_cache_entry(page_cache);
                __add_to_page_cache(page, inode, pos & PAGE_CACHE_MASK, hash);
                spin_unlock(&pagecache_lock);

                page_cache = 0;
                goto readpage;
        }

        *ppos = pos;
        filp->f_reada = 1;
        if (page_cache)
                page_cache_free(page_cache);
        UPDATE_ATIME(inode);
}

static int file_read_actor(read_descriptor_t * desc, const char *area, unsigned long size)
{
        unsigned long left;
        unsigned long count = desc->count;

        if (size > count)
                size = count;
        left = __copy_to_user(desc->buf, area, size);
        if (left) {
                size -= left;
                desc->error = -EFAULT;
        }
        desc->count = count - size;
        desc->written += size;
        desc->buf += size;
        return size;
}

/*
 * This is the "read()" routine for all filesystems
 * that can use the page cache directly.
 */
ssize_t generic_file_read(struct file * filp, char * buf, size_t count, loff_t *ppos)
{
        ssize_t retval;

        retval = -EFAULT;
        if (access_ok(VERIFY_WRITE, buf, count)) {
                retval = 0;
                if (count) {
                        read_descriptor_t desc;

                        desc.written = 0;
                        desc.count = count;
                        desc.buf = buf;
                        desc.error = 0;
                        do_generic_file_read(filp, ppos, &desc, file_read_actor);

                        retval = desc.written;
                        if (!retval)
                                retval = desc.error;
                }
        }
        return retval;
}

static int file_send_actor(read_descriptor_t * desc, const char *area, unsigned long size)
{
        ssize_t written;
        unsigned long count = desc->count;
        struct file *file = (struct file *) desc->buf;
        mm_segment_t old_fs;

        if (size > count)
                size = count;
        old_fs = get_fs();
        set_fs(KERNEL_DS);
        written = file->f_op->write(file, area, size, &file->f_pos);
        set_fs(old_fs);
        if (written < 0) {
                desc->error = written;
                written = 0;
        }
        desc->count = count - written;
        desc->written += written;
        return written;
}

asmlinkage ssize_t sys_sendfile(int out_fd, int in_fd, off_t *offset, size_t count)
{
        ssize_t retval;
        struct file * in_file, * out_file;
        struct inode * in_inode, * out_inode;

        /*
         * Get input file, and verify that it is ok..
         */
        retval = -EBADF;
        in_file = fget(in_fd);
        if (!in_file)
                goto out;
        if (!(in_file->f_mode & FMODE_READ))
                goto fput_in;
        retval = -EINVAL;
        in_inode = in_file->f_dentry->d_inode;
        if (!in_inode)
                goto fput_in;
        if (!in_inode->i_op || !in_inode->i_op->readpage)
                goto fput_in;
        retval = locks_verify_area(FLOCK_VERIFY_READ, in_inode, in_file, in_file->f_pos, count);
        if (retval)
                goto fput_in;

        /*
         * Get output file, and verify that it is ok..
         */
        retval = -EBADF;
        out_file = fget(out_fd);
        if (!out_file)
                goto fput_in;
        if (!(out_file->f_mode & FMODE_WRITE))
                goto fput_out;
        retval = -EINVAL;
        if (!out_file->f_op || !out_file->f_op->write)
                goto fput_out;
        out_inode = out_file->f_dentry->d_inode;
        if (!out_inode)
                goto fput_out;
        retval = locks_verify_area(FLOCK_VERIFY_WRITE, out_inode, out_file, out_file->f_pos, count);
        if (retval)
                goto fput_out;

        retval = 0;
        if (count) {
                read_descriptor_t desc;
                loff_t pos = 0, *ppos;

                retval = -EFAULT;
                ppos = &in_file->f_pos;
                if (offset) {
                        if (get_user(pos, offset))
                                goto fput_out;
                        ppos = &pos;
                }

                desc.written = 0;
                desc.count = count;
                desc.buf = (char *) out_file;
                desc.error = 0;
                do_generic_file_read(in_file, ppos, &desc, file_send_actor);

                retval = desc.written;
                if (!retval)
                        retval = desc.error;
                if (offset)
                        put_user(pos, offset);
        }

fput_out:
        fput(out_file);
fput_in:
        fput(in_file);
out:
        return retval;
}

/*
 * Semantics for shared and private memory areas are different past the end
 * of the file. A shared mapping past the last page of the file is an error
 * and results in a SIGBUS, while a private mapping just maps in a zero page.
 *
 * The goto's are kind of ugly, but this streamlines the normal case of having
 * it in the page cache, and handles the special cases reasonably without
 * having a lot of duplicated code.
 *
 * WSH 06/04/97: fixed a memory leak and moved the allocation of new_page
 * ahead of the wait if we're sure to need it.
 */
static unsigned long filemap_nopage(struct vm_area_struct * area, unsigned long address, int no_share)
{
        struct file * file = area->vm_file;
        struct dentry * dentry = file->f_dentry;
        struct inode * inode = dentry->d_inode;
        unsigned long offset, reada, i;
        struct page * page, **hash;
        unsigned long old_page, new_page;
        int error;

        new_page = 0;
        offset = (address & PAGE_MASK) - area->vm_start + area->vm_offset;
        if (offset >= inode->i_size && (area->vm_flags & VM_SHARED) && area->vm_mm == current->mm)
                goto no_page;

        /*
         * Do we have something in the page cache already?
         */
        hash = page_hash(inode, offset);
retry_find:
        page = __find_get_page(inode, offset, hash);
        if (!page)
                goto no_cached_page;

found_page:
        /*
         * Ok, found a page in the page cache, now we need to check
         * that it's up-to-date.  First check whether we'll need an
         * extra page -- better to overlap the allocation with the I/O.
         */
        if (no_share && !new_page) {
                new_page = page_cache_alloc();
                if (!new_page)
                        goto failure;
        }

        if (!Page_Uptodate(page)) {
                lock_page(page);
                if (!Page_Uptodate(page))
                        goto page_not_uptodate;
                UnlockPage(page);
        }

success:
        /*
         * Found the page and have a reference on it, need to check sharing
         * and possibly copy it over to another page..
         */
        old_page = page_address(page);
        if (!no_share) {
                /*
                 * Ok, we can share the cached page directly.. Get rid
                 * of any potential extra pages.
                 */
                if (new_page)
                        page_cache_free(new_page);

                flush_page_to_ram(old_page);
                return old_page;
        }

        /*
         * No sharing ... copy to the new page.
         */
        copy_page(new_page, old_page);
        flush_page_to_ram(new_page);
        page_cache_release(page);
        return new_page;

no_cached_page:
        /*
         * Try to read in an entire cluster at once.
         */
        reada   = offset;
        reada >>= PAGE_CACHE_SHIFT + page_cluster;
        reada <<= PAGE_CACHE_SHIFT + page_cluster;

        for (i = 1 << page_cluster; i > 0; --i, reada += PAGE_CACHE_SIZE)
                new_page = try_to_read_ahead(file, reada, new_page);

        if (!new_page)
                new_page = page_cache_alloc();
        if (!new_page)
                goto no_page;

        /*
         * During getting the above page we might have slept,
         * so we need to re-check the situation with the page
         * cache.. The page we just got may be useful if we
         * can't share, so don't get rid of it here.
         */
        page = __find_get_page(inode, offset, hash);
        if (page)
                goto found_page;

        /*
         * Now, create a new page-cache page from the page we got
         */
        page = page_cache_entry(new_page);
        if (add_to_page_cache_unique(page, inode, offset, hash))
                goto retry_find;

        /*
         * Now it's ours and locked, we can do initial IO to it:
         */
        new_page = 0;

page_not_uptodate:
        error = inode->i_op->readpage(file, page);

        if (!error) {
                wait_on_page(page);
                if (PageError(page))
                        goto page_read_error;
                goto success;
        }

page_read_error:
        /*
         * Umm, take care of errors if the page isn't up-to-date.
         * Try to re-read it _once_. We do this synchronously,
         * because there really aren't any performance issues here
         * and we need to check for errors.
         */
        if (!PageLocked(page))
                PAGE_BUG(page);
        ClearPageError(page);
        error = inode->i_op->readpage(file, page);
        if (error)
                goto failure;
        wait_on_page(page);
        if (Page_Uptodate(page))
                goto success;

        /*
         * Things didn't work out. Return zero to tell the
         * mm layer so, possibly freeing the page cache page first.
         */
failure:
        page_cache_release(page);
        if (new_page)
                page_cache_free(new_page);
no_page:
        return 0;
}

/*
 * Tries to write a shared mapped page to its backing store. May return -EIO
 * if the disk is full.
 */
static inline int do_write_page(struct inode * inode, struct file * file,
        const char * page_addr, unsigned long offset)
{
        int retval;
        unsigned long size;
        int (*writepage) (struct file *, struct page *);
        struct page * page;

        size = offset + PAGE_SIZE;
        /* refuse to extend file size.. */
        if (S_ISREG(inode->i_mode)) {
                if (size > inode->i_size)
                        size = inode->i_size;
                /* Ho humm.. We should have tested for this earlier */
                if (size < offset)
                        return -EIO;
        }
        size -= offset;
        retval = -EIO;
        writepage = inode->i_op->writepage;
        page = mem_map + MAP_NR(page_addr);
        lock_page(page);

        retval = writepage(file, page);

        UnlockPage(page);
        return retval;
}

static int filemap_write_page(struct vm_area_struct * vma,
                              unsigned long offset,
                              unsigned long page,
                              int wait)
{
        int result;
        struct file * file;
        struct dentry * dentry;
        struct inode * inode;

        file = vma->vm_file;
        dentry = file->f_dentry;
        inode = dentry->d_inode;

        /*
         * If a task terminates while we're swapping the page, the vma and
         * and file could be released ... increment the count to be safe.
         */
        get_file(file);
        result = do_write_page(inode, file, (const char *) page, offset);
        fput(file);
        return result;
}


/*
 * The page cache takes care of races between somebody
 * trying to swap something out and swap something in
 * at the same time..
 */
extern void wakeup_bdflush(int);
int filemap_swapout(struct vm_area_struct * vma, struct page * page)
{
        int retval = filemap_write_page(vma, page->offset, page_address(page), 0);
        wakeup_bdflush(0);
        return retval;
}

static inline int filemap_sync_pte(pte_t * ptep, struct vm_area_struct *vma,
        unsigned long address, unsigned int flags)
{
        pte_t pte = *ptep;
        unsigned long pageaddr;
        struct page *page;
        int error;

        if (!(flags & MS_INVALIDATE)) {
                if (!pte_present(pte))
                        return 0;
                if (!pte_dirty(pte))
                        return 0;
                flush_page_to_ram(pte_page(pte));
                flush_cache_page(vma, address);
                set_pte(ptep, pte_mkclean(pte));
                flush_tlb_page(vma, address);
                pageaddr = pte_page(pte);
                page = page_cache_entry(pageaddr);
                get_page(page);
        } else {
                if (pte_none(pte))
                        return 0;
                flush_cache_page(vma, address);
                pte_clear(ptep);
                flush_tlb_page(vma, address);
                if (!pte_present(pte)) {
                        swap_free(pte_val(pte));
                        return 0;
                }
                pageaddr = pte_page(pte);
                if (!pte_dirty(pte) || flags == MS_INVALIDATE) {
                        page_cache_free(pageaddr);
                        return 0;
                }
        }
        error = filemap_write_page(vma, address - vma->vm_start + vma->vm_offset, pageaddr, 1);
        page_cache_free(pageaddr);
        return error;
}

static inline int filemap_sync_pte_range(pmd_t * pmd,
        unsigned long address, unsigned long size, 
        struct vm_area_struct *vma, unsigned long offset, unsigned int flags)
{
        pte_t * pte;
        unsigned long end;
        int error;

        if (pmd_none(*pmd))
                return 0;
        if (pmd_bad(*pmd)) {
                printk("filemap_sync_pte_range: bad pmd (%08lx)\n", pmd_val(*pmd));
                pmd_clear(pmd);
                return 0;
        }
        pte = pte_offset(pmd, address);
        offset += address & PMD_MASK;
        address &= ~PMD_MASK;
        end = address + size;
        if (end > PMD_SIZE)
                end = PMD_SIZE;
        error = 0;
        do {
                error |= filemap_sync_pte(pte, vma, address + offset, flags);
                address += PAGE_SIZE;
                pte++;
        } while (address < end);
        return error;
}

static inline int filemap_sync_pmd_range(pgd_t * pgd,
        unsigned long address, unsigned long size, 
        struct vm_area_struct *vma, unsigned int flags)
{
        pmd_t * pmd;
        unsigned long offset, end;
        int error;

        if (pgd_none(*pgd))
                return 0;
        if (pgd_bad(*pgd)) {
                printk("filemap_sync_pmd_range: bad pgd (%08lx)\n", pgd_val(*pgd));
                pgd_clear(pgd);
                return 0;
        }
        pmd = pmd_offset(pgd, address);
        offset = address & PGDIR_MASK;
        address &= ~PGDIR_MASK;
        end = address + size;
        if (end > PGDIR_SIZE)
                end = PGDIR_SIZE;
        error = 0;
        do {
                error |= filemap_sync_pte_range(pmd, address, end - address, vma, offset, flags);
                address = (address + PMD_SIZE) & PMD_MASK;
                pmd++;
        } while (address < end);
        return error;
}

static int filemap_sync(struct vm_area_struct * vma, unsigned long address,
        size_t size, unsigned int flags)
{
        pgd_t * dir;
        unsigned long end = address + size;
        int error = 0;

        dir = pgd_offset(vma->vm_mm, address);
        flush_cache_range(vma->vm_mm, end - size, end);
        while (address < end) {
                error |= filemap_sync_pmd_range(dir, address, end - address, vma, flags);
                address = (address + PGDIR_SIZE) & PGDIR_MASK;
                dir++;
        }
        flush_tlb_range(vma->vm_mm, end - size, end);
        return error;
}

/*
 * This handles (potentially partial) area unmaps..
 */
static void filemap_unmap(struct vm_area_struct *vma, unsigned long start, size_t len)
{
        filemap_sync(vma, start, len, MS_ASYNC);
}

/*
 * Shared mappings need to be able to do the right thing at
 * close/unmap/sync. They will also use the private file as
 * backing-store for swapping..
 */
static struct vm_operations_struct file_shared_mmap = {
        NULL,                   /* no special open */
        NULL,                   /* no special close */
        filemap_unmap,          /* unmap - we need to sync the pages */
        NULL,                   /* no special protect */
        filemap_sync,           /* sync */
        NULL,                   /* advise */
        filemap_nopage,         /* nopage */
        NULL,                   /* wppage */
        filemap_swapout         /* swapout */
};

/*
 * Private mappings just need to be able to load in the map.
 *
 * (This is actually used for shared mappings as well, if we
 * know they can't ever get write permissions..)
 */
static struct vm_operations_struct file_private_mmap = {
        NULL,                   /* open */
        NULL,                   /* close */
        NULL,                   /* unmap */
        NULL,                   /* protect */
        NULL,                   /* sync */
        NULL,                   /* advise */
        filemap_nopage,         /* nopage */
        NULL,                   /* wppage */
        NULL                    /* swapout */
};

/* This is used for a general mmap of a disk file */

int generic_file_mmap(struct file * file, struct vm_area_struct * vma)
{
        struct vm_operations_struct * ops;
        struct inode *inode = file->f_dentry->d_inode;

        ops = &file_private_mmap;
        if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE)) {
                if (!inode->i_op || !inode->i_op->writepage)
                        return -EINVAL;
                ops = &file_shared_mmap;
        }
        if (!inode->i_sb || !S_ISREG(inode->i_mode))
                return -EACCES;
        if (!inode->i_op || !inode->i_op->readpage)
                return -ENOEXEC;
        UPDATE_ATIME(inode);
        vma->vm_ops = ops;
        return 0;
}


/*
 * The msync() system call.
 */

static int msync_interval(struct vm_area_struct * vma,
        unsigned long start, unsigned long end, int flags)
{
        if (vma->vm_file && vma->vm_ops && vma->vm_ops->sync) {
                int error;
                error = vma->vm_ops->sync(vma, start, end-start, flags);
                if (!error && (flags & MS_SYNC)) {
                        struct file * file = vma->vm_file;
                        if (file) {
                                struct dentry * dentry = file->f_dentry;
                                error = file_fsync(file, dentry);
                        }
                }
                return error;
        }
        return 0;
}

asmlinkage int sys_msync(unsigned long start, size_t len, int flags)
{
        unsigned long end;
        struct vm_area_struct * vma;
        int unmapped_error, error = -EINVAL;

        down(&current->mm->mmap_sem);
        lock_kernel();
        if (start & ~PAGE_MASK)
                goto out;
        len = (len + ~PAGE_MASK) & PAGE_MASK;
        end = start + len;
        if (end < start)
                goto out;
        if (flags & ~(MS_ASYNC | MS_INVALIDATE | MS_SYNC))
                goto out;
        error = 0;
        if (end == start)
                goto out;
        /*
         * If the interval [start,end) covers some unmapped address ranges,
         * just ignore them, but return -EFAULT at the end.
         */
        vma = find_vma(current->mm, start);
        unmapped_error = 0;
        for (;;) {
                /* Still start < end. */
                error = -EFAULT;
                if (!vma)
                        goto out;
                /* Here start < vma->vm_end. */
                if (start < vma->vm_start) {
                        unmapped_error = -EFAULT;
                        start = vma->vm_start;
                }
                /* Here vma->vm_start <= start < vma->vm_end. */
                if (end <= vma->vm_end) {
                        if (start < end) {
                                error = msync_interval(vma, start, end, flags);
                                if (error)
                                        goto out;
                        }
                        error = unmapped_error;
                        goto out;
                }
                /* Here vma->vm_start <= start < vma->vm_end < end. */
                error = msync_interval(vma, start, vma->vm_end, flags);
                if (error)
                        goto out;
                start = vma->vm_end;
                vma = vma->vm_next;
        }
out:
        unlock_kernel();
        up(&current->mm->mmap_sem);
        return error;
}

/*
 * Write to a file through the page cache. This is mainly for the
 * benefit of NFS and possibly other network-based file systems.
 *
 * We currently put everything into the page cache prior to writing it.
 * This is not a problem when writing full pages. With partial pages,
 * however, we first have to read the data into the cache, then
 * dirty the page, and finally schedule it for writing. Alternatively, we
 * could write-through just the portion of data that would go into that
 * page, but that would kill performance for applications that write data
 * line by line, and it's prone to race conditions.
 *
 * Note that this routine doesn't try to keep track of dirty pages. Each
 * file system has to do this all by itself, unfortunately.
 *                                                      okir@monad.swb.de
 */
ssize_t
generic_file_write(struct file *file, const char *buf,
                   size_t count, loff_t *ppos,
                   writepage_t write_one_page)
{
        struct dentry   *dentry = file->f_dentry; 
        struct inode    *inode = dentry->d_inode; 
        unsigned long   pos = *ppos;
        unsigned long   limit = current->rlim[RLIMIT_FSIZE].rlim_cur;
        struct page     *page, **hash;
        unsigned long   page_cache = 0;
        unsigned long   written;
        long            status;
        int             err;

        err = file->f_error;
        if (err) {
                file->f_error = 0;
                goto out;
        }

        written = 0;

        if (file->f_flags & O_APPEND)
                pos = inode->i_size;

        /*
         * Check whether we've reached the file size limit.
         */
        err = -EFBIG;
        if (pos >= limit) {
                send_sig(SIGXFSZ, current, 0);
                goto out;
        }

        status  = 0;
        /*
         * Check whether to truncate the write,
         * and send the signal if we do.
         */
        if (count > limit - pos) {
                send_sig(SIGXFSZ, current, 0);
                count = limit - pos;
        }

        while (count) {
                unsigned long bytes, pgpos, offset;
                /*
                 * Try to find the page in the cache. If it isn't there,
                 * allocate a free page.
                 */
                offset = (pos & ~PAGE_CACHE_MASK);
                pgpos = pos & PAGE_CACHE_MASK;
                bytes = PAGE_CACHE_SIZE - offset;
                if (bytes > count)
                        bytes = count;

                hash = page_hash(inode, pgpos);
repeat_find:
                page = __find_lock_page(inode, pgpos, hash);
                if (!page) {
                        if (!page_cache) {
                                page_cache = page_cache_alloc();
                                if (page_cache)
                                        goto repeat_find;
                                status = -ENOMEM;
                                break;
                        }
                        page = page_cache_entry(page_cache);
                        if (add_to_page_cache_unique(page,inode,pgpos,hash))
                                goto repeat_find;

                        page_cache = 0;
                }

                /* We have exclusive IO access to the page.. */
                if (!PageLocked(page)) {
                        PAGE_BUG(page);
                } else {
                        if (page->owner != current) {
                                PAGE_BUG(page);
                        }
                }

                status = write_one_page(file, page, offset, bytes, buf);

                /* Mark it unlocked again and drop the page.. */
                UnlockPage(page);
                page_cache_release(page);

                if (status < 0)
                        break;

                written += status;
                count -= status;
                pos += status;
                buf += status;
        }
        *ppos = pos;
        if (pos > inode->i_size)
                inode->i_size = pos;

        if (page_cache)
                page_cache_free(page_cache);

        err = written ? written : status;
out:
        return err;
}

/*
 * Support routines for directory caching using the page cache.
 */

/*
 * Unlock and free a page.
 */
void put_cached_page(unsigned long addr)
{
        struct page * page = page_cache_entry(addr);

        UnlockPage(page);
        if (page_count(page) != 2)
                panic("put_cached_page: page count=%d\n", 
                        page_count(page));
        page_cache_release(page);
}

void __init page_cache_init(unsigned long memory_size)
{
        unsigned long htable_size, order;

        htable_size = memory_size >> PAGE_SHIFT;
        htable_size *= sizeof(struct page *);
        for(order = 0; (PAGE_SIZE << order) < htable_size; order++)
                ;

        do {
                unsigned long tmp = (PAGE_SIZE << order) / sizeof(struct page *);

                page_hash_bits = 0;
                while((tmp >>= 1UL) != 0UL)
                        page_hash_bits++;

                page_hash_table = (struct page **)
                        __get_free_pages(GFP_ATOMIC, order);
        } while(page_hash_table == NULL && --order > 0);

        printk("Page-cache hash table entries: %d (order: %ld, %ld bytes)\n",
               (1 << page_hash_bits), order, (PAGE_SIZE << order));
        if (!page_hash_table)
                panic("Failed to allocate page hash table\n");
        memset(page_hash_table, 0, PAGE_HASH_SIZE * sizeof(struct page *));
}