Add to mips64 also for symmetry.

[linux-2.6/linux-mips.git] / fs / direct-io.c

/*
 * fs/direct-io.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * O_DIRECT
 *
 * 04Jul2002    akpm@zip.com.au
 *              Initial version
 * 11Sep2002    janetinc@us.ibm.com
 *              added readv/writev support.
 * 29Oct2002    akpm@zip.com.au
 *              rewrote bio_add_page() support.
 * 30Oct2002    pbadari@us.ibm.com
 *              added support for non-aligned IO.
 * 06Nov2002    pbadari@us.ibm.com
 *              added asynchronous IO support.
 */

#include <linux/kernel.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>
#include <linux/bio.h>
#include <linux/wait.h>
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>
#include <linux/rwsem.h>
#include <linux/uio.h>
#include <asm/atomic.h>

/*
 * How many user pages to map in one call to get_user_pages().  This determines
 * the size of a structure on the stack.
 */
#define DIO_PAGES       64

/*
 * This code generally works in units of "dio_blocks".  A dio_block is
 * somewhere between the hard sector size and the filesystem block size.  it
 * is determined on a per-invocation basis.   When talking to the filesystem
 * we need to convert dio_blocks to fs_blocks by scaling the dio_block quantity
 * down by dio->blkfactor.  Similarly, fs-blocksize quantities are converted
 * to bio_block quantities by shifting left by blkfactor.
 *
 * If blkfactor is zero then the user's request was aligned to the filesystem's
 * blocksize.
 */

struct dio {
        /* BIO submission state */
        struct bio *bio;                /* bio under assembly */
        struct inode *inode;
        int rw;
        unsigned blkbits;               /* doesn't change */
        unsigned blkfactor;             /* When we're using an alignment which
                                           is finer than the filesystem's soft
                                           blocksize, this specifies how much
                                           finer.  blkfactor=2 means 1/4-block
                                           alignment.  Does not change */
        unsigned start_zero_done;       /* flag: sub-blocksize zeroing has
                                           been performed at the start of a
                                           write */
        int pages_in_io;                /* approximate total IO pages */
        sector_t block_in_file;         /* Current offset into the underlying
                                           file in dio_block units. */
        unsigned blocks_available;      /* At block_in_file.  changes */
        sector_t final_block_in_request;/* doesn't change */
        unsigned first_block_in_page;   /* doesn't change, Used only once */
        int boundary;                   /* prev block is at a boundary */
        int reap_counter;               /* rate limit reaping */
        get_blocks_t *get_blocks;       /* block mapping function */
        sector_t final_block_in_bio;    /* current final block in bio + 1 */
        sector_t next_block_for_io;     /* next block to be put under IO,
                                           in dio_blocks units */
        struct buffer_head map_bh;      /* last get_blocks() result */

        /*
         * Deferred addition of a page to the dio.  These variables are
         * private to dio_send_cur_page(), submit_page_section() and
         * dio_bio_add_page().
         */
        struct page *cur_page;          /* The page */
        unsigned cur_page_offset;       /* Offset into it, in bytes */
        unsigned cur_page_len;          /* Nr of bytes at cur_page_offset */
        sector_t cur_page_block;        /* Where it starts */

        /*
         * Page fetching state. These variables belong to dio_refill_pages().
         */
        int curr_page;                  /* changes */
        int total_pages;                /* doesn't change */
        unsigned long curr_user_address;/* changes */

        /*
         * Page queue.  These variables belong to dio_refill_pages() and
         * dio_get_page().
         */
        struct page *pages[DIO_PAGES];  /* page buffer */
        unsigned head;                  /* next page to process */
        unsigned tail;                  /* last valid page + 1 */
        int page_errors;                /* errno from get_user_pages() */

        /* BIO completion state */
        atomic_t bio_count;             /* nr bios to be completed */
        atomic_t bios_in_flight;        /* nr bios in flight */
        spinlock_t bio_list_lock;       /* protects bio_list */
        struct bio *bio_list;           /* singly linked via bi_private */
        struct task_struct *waiter;     /* waiting task (NULL if none) */

        /* AIO related stuff */
        struct kiocb *iocb;             /* kiocb */
        int is_async;                   /* is IO async ? */
        int result;                     /* IO result */
};

/*
 * How many pages are in the queue?
 */
static inline unsigned dio_pages_present(struct dio *dio)
{
        return dio->tail - dio->head;
}

/*
 * Go grab and pin some userspace pages.   Typically we'll get 64 at a time.
 */
static int dio_refill_pages(struct dio *dio)
{
        int ret;
        int nr_pages;

        nr_pages = min(dio->total_pages - dio->curr_page, DIO_PAGES);
        down_read(&current->mm->mmap_sem);
        ret = get_user_pages(
                current,                        /* Task for fault acounting */
                current->mm,                    /* whose pages? */
                dio->curr_user_address,         /* Where from? */
                nr_pages,                       /* How many pages? */
                dio->rw == READ,                /* Write to memory? */
                0,                              /* force (?) */
                &dio->pages[0],
                NULL);                          /* vmas */
        up_read(&current->mm->mmap_sem);

        if (ret < 0 && dio->blocks_available && (dio->rw == WRITE)) {
                /*
                 * A memory fault, but the filesystem has some outstanding
                 * mapped blocks.  We need to use those blocks up to avoid
                 * leaking stale data in the file.
                 */
                if (dio->page_errors == 0)
                        dio->page_errors = ret;
                dio->pages[0] = ZERO_PAGE(dio->curr_user_address);
                dio->head = 0;
                dio->tail = 1;
                ret = 0;
                goto out;
        }

        if (ret >= 0) {
                dio->curr_user_address += ret * PAGE_SIZE;
                dio->curr_page += ret;
                dio->head = 0;
                dio->tail = ret;
                ret = 0;
        }
out:
        return ret;     
}

/*
 * Get another userspace page.  Returns an ERR_PTR on error.  Pages are
 * buffered inside the dio so that we can call get_user_pages() against a
 * decent number of pages, less frequently.  To provide nicer use of the
 * L1 cache.
 */
static struct page *dio_get_page(struct dio *dio)
{
        if (dio_pages_present(dio) == 0) {
                int ret;

                ret = dio_refill_pages(dio);
                if (ret)
                        return ERR_PTR(ret);
                BUG_ON(dio_pages_present(dio) == 0);
        }
        return dio->pages[dio->head++];
}

/*
 * Called when a BIO has been processed.  If the count goes to zero then IO is
 * complete and we can signal this to the AIO layer.
 */
static void finished_one_bio(struct dio *dio)
{
        if (atomic_dec_and_test(&dio->bio_count)) {
                if(dio->is_async) {
                        aio_complete(dio->iocb, dio->result, 0);
                        kfree(dio);
                }
        }
}

static int dio_bio_complete(struct dio *dio, struct bio *bio);
/*
 * Asynchronous IO callback. 
 */
static int dio_bio_end_aio(struct bio *bio, unsigned int bytes_done, int error)
{
        struct dio *dio = bio->bi_private;

        if (bio->bi_size)
                return 1;

        /* cleanup the bio */
        dio_bio_complete(dio, bio);
        return 0;
}

/*
 * The BIO completion handler simply queues the BIO up for the process-context
 * handler.
 *
 * During I/O bi_private points at the dio.  After I/O, bi_private is used to
 * implement a singly-linked list of completed BIOs, at dio->bio_list.
 */
static int dio_bio_end_io(struct bio *bio, unsigned int bytes_done, int error)
{
        struct dio *dio = bio->bi_private;
        unsigned long flags;

        if (bio->bi_size)
                return 1;

        spin_lock_irqsave(&dio->bio_list_lock, flags);
        bio->bi_private = dio->bio_list;
        dio->bio_list = bio;
        atomic_dec(&dio->bios_in_flight);
        if (dio->waiter && atomic_read(&dio->bios_in_flight) == 0)
                wake_up_process(dio->waiter);
        spin_unlock_irqrestore(&dio->bio_list_lock, flags);
        return 0;
}

static int
dio_bio_alloc(struct dio *dio, struct block_device *bdev,
                sector_t first_sector, int nr_vecs)
{
        struct bio *bio;

        bio = bio_alloc(GFP_KERNEL, nr_vecs);
        if (bio == NULL)
                return -ENOMEM;

        bio->bi_bdev = bdev;
        bio->bi_sector = first_sector;
        if (dio->is_async)
                bio->bi_end_io = dio_bio_end_aio;
        else
                bio->bi_end_io = dio_bio_end_io;

        dio->bio = bio;
        return 0;
}

/*
 * In the AIO read case we speculatively dirty the pages before starting IO.
 * During IO completion, any of these pages which happen to have been written
 * back will be redirtied by bio_check_pages_dirty().
 */
static void dio_bio_submit(struct dio *dio)
{
        struct bio *bio = dio->bio;

        bio->bi_private = dio;
        atomic_inc(&dio->bio_count);
        atomic_inc(&dio->bios_in_flight);
        if (dio->is_async && dio->rw == READ)
                bio_set_pages_dirty(bio);
        submit_bio(dio->rw, bio);

        dio->bio = NULL;
        dio->boundary = 0;
}

/*
 * Release any resources in case of a failure
 */
static void dio_cleanup(struct dio *dio)
{
        while (dio_pages_present(dio))
                page_cache_release(dio_get_page(dio));
}

/*
 * Wait for the next BIO to complete.  Remove it and return it.
 */
static struct bio *dio_await_one(struct dio *dio)
{
        unsigned long flags;
        struct bio *bio;

        spin_lock_irqsave(&dio->bio_list_lock, flags);
        while (dio->bio_list == NULL) {
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (dio->bio_list == NULL) {
                        dio->waiter = current;
                        spin_unlock_irqrestore(&dio->bio_list_lock, flags);
                        blk_run_queues();
                        io_schedule();
                        spin_lock_irqsave(&dio->bio_list_lock, flags);
                        dio->waiter = NULL;
                }
                set_current_state(TASK_RUNNING);
        }
        bio = dio->bio_list;
        dio->bio_list = bio->bi_private;
        spin_unlock_irqrestore(&dio->bio_list_lock, flags);
        return bio;
}

/*
 * Process one completed BIO.  No locks are held.
 */
static int dio_bio_complete(struct dio *dio, struct bio *bio)
{
        const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
        struct bio_vec *bvec = bio->bi_io_vec;
        int page_no;

        if (!uptodate)
                dio->result = -EIO;

        if (dio->is_async && dio->rw == READ) {
                bio_check_pages_dirty(bio);     /* transfers ownership */
        } else {
                for (page_no = 0; page_no < bio->bi_vcnt; page_no++) {
                        struct page *page = bvec[page_no].bv_page;

                        if (dio->rw == READ)
                                set_page_dirty_lock(page);
                        page_cache_release(page);
                }
                bio_put(bio);
        }
        finished_one_bio(dio);
        return uptodate ? 0 : -EIO;
}

/*
 * Wait on and process all in-flight BIOs.
 */
static int dio_await_completion(struct dio *dio)
{
        int ret = 0;

        if (dio->bio)
                dio_bio_submit(dio);

        while (atomic_read(&dio->bio_count)) {
                struct bio *bio = dio_await_one(dio);
                int ret2;

                ret2 = dio_bio_complete(dio, bio);
                if (ret == 0)
                        ret = ret2;
        }
        return ret;
}

/*
 * A really large O_DIRECT read or write can generate a lot of BIOs.  So
 * to keep the memory consumption sane we periodically reap any completed BIOs
 * during the BIO generation phase.
 *
 * This also helps to limit the peak amount of pinned userspace memory.
 */
static int dio_bio_reap(struct dio *dio)
{
        int ret = 0;

        if (dio->reap_counter++ >= 64) {
                while (dio->bio_list) {
                        unsigned long flags;
                        struct bio *bio;

                        spin_lock_irqsave(&dio->bio_list_lock, flags);
                        bio = dio->bio_list;
                        dio->bio_list = bio->bi_private;
                        spin_unlock_irqrestore(&dio->bio_list_lock, flags);
                        ret = dio_bio_complete(dio, bio);
                }
                dio->reap_counter = 0;
        }
        return ret;
}

/*
 * Call into the fs to map some more disk blocks.  We record the current number
 * of available blocks at dio->blocks_available.  These are in units of the
 * fs blocksize, (1 << inode->i_blkbits).
 *
 * The fs is allowed to map lots of blocks at once.  If it wants to do that,
 * it uses the passed inode-relative block number as the file offset, as usual.
 *
 * get_blocks() is passed the number of i_blkbits-sized blocks which direct_io
 * has remaining to do.  The fs should not map more than this number of blocks.
 *
 * If the fs has mapped a lot of blocks, it should populate bh->b_size to
 * indicate how much contiguous disk space has been made available at
 * bh->b_blocknr.
 *
 * If *any* of the mapped blocks are new, then the fs must set buffer_new().
 * This isn't very efficient...
 *
 * In the case of filesystem holes: the fs may return an arbitrarily-large
 * hole by returning an appropriate value in b_size and by clearing
 * buffer_mapped().  However the direct-io code will only process holes one
 * block at a time - it will repeatedly call get_blocks() as it walks the hole.
 */
static int get_more_blocks(struct dio *dio)
{
        int ret;
        struct buffer_head *map_bh = &dio->map_bh;
        sector_t fs_startblk;   /* Into file, in filesystem-sized blocks */
        unsigned long fs_count; /* Number of filesystem-sized blocks */
        unsigned long dio_count;/* Number of dio_block-sized blocks */
        unsigned long blkmask;

        /*
         * If there was a memory error and we've overwritten all the
         * mapped blocks then we can now return that memory error
         */
        ret = dio->page_errors;
        if (ret == 0) {
                map_bh->b_state = 0;
                map_bh->b_size = 0;
                BUG_ON(dio->block_in_file >= dio->final_block_in_request);
                fs_startblk = dio->block_in_file >> dio->blkfactor;
                dio_count = dio->final_block_in_request - dio->block_in_file;
                fs_count = dio_count >> dio->blkfactor;
                blkmask = (1 << dio->blkfactor) - 1;
                if (dio_count & blkmask)        
                        fs_count++;

                ret = (*dio->get_blocks)(dio->inode, fs_startblk, fs_count,
                                map_bh, dio->rw == WRITE);
        }
        return ret;
}

/*
 * There is no bio.  Make one now.
 */
static int dio_new_bio(struct dio *dio, sector_t start_sector)
{
        sector_t sector;
        int ret, nr_pages;

        ret = dio_bio_reap(dio);
        if (ret)
                goto out;
        sector = start_sector << (dio->blkbits - 9);
        nr_pages = min(dio->pages_in_io, bio_get_nr_vecs(dio->map_bh.b_bdev));
        BUG_ON(nr_pages <= 0);
        ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
        dio->boundary = 0;
out:
        return ret;
}

/*
 * Attempt to put the current chunk of 'cur_page' into the current BIO.  If
 * that was successful then update final_block_in_bio and take a ref against
 * the just-added page.
 *
 * Return zero on success.  Non-zero means the caller needs to start a new BIO.
 */
static int dio_bio_add_page(struct dio *dio)
{
        int ret;

        ret = bio_add_page(dio->bio, dio->cur_page,
                        dio->cur_page_len, dio->cur_page_offset);
        if (ret == dio->cur_page_len) {
                dio->pages_in_io--;
                page_cache_get(dio->cur_page);
                dio->final_block_in_bio = dio->cur_page_block +
                        (dio->cur_page_len >> dio->blkbits);
                ret = 0;
        } else {
                ret = 1;
        }
        return ret;
}
                
/*
 * Put cur_page under IO.  The section of cur_page which is described by
 * cur_page_offset,cur_page_len is put into a BIO.  The section of cur_page
 * starts on-disk at cur_page_block.
 *
 * We take a ref against the page here (on behalf of its presence in the bio).
 *
 * The caller of this function is responsible for removing cur_page from the
 * dio, and for dropping the refcount which came from that presence.
 */
static int dio_send_cur_page(struct dio *dio)
{
        int ret = 0;

        if (dio->bio) {
                /*
                 * See whether this new request is contiguous with the old
                 */
                if (dio->final_block_in_bio != dio->cur_page_block)
                        dio_bio_submit(dio);
                /*
                 * Submit now if the underlying fs is about to perform a
                 * metadata read
                 */
                if (dio->boundary)
                        dio_bio_submit(dio);
        }

        if (dio->bio == NULL) {
                ret = dio_new_bio(dio, dio->cur_page_block);
                if (ret)
                        goto out;
        }

        if (dio_bio_add_page(dio) != 0) {
                dio_bio_submit(dio);
                ret = dio_new_bio(dio, dio->cur_page_block);
                if (ret == 0) {
                        ret = dio_bio_add_page(dio);
                        BUG_ON(ret != 0);
                }
        }
out:
        return ret;
}

/*
 * An autonomous function to put a chunk of a page under deferred IO.
 *
 * The caller doesn't actually know (or care) whether this piece of page is in
 * a BIO, or is under IO or whatever.  We just take care of all possible 
 * situations here.  The separation between the logic of do_direct_IO() and
 * that of submit_page_section() is important for clarity.  Please don't break.
 *
 * The chunk of page starts on-disk at blocknr.
 *
 * We perform deferred IO, by recording the last-submitted page inside our
 * private part of the dio structure.  If possible, we just expand the IO
 * across that page here.
 *
 * If that doesn't work out then we put the old page into the bio and add this
 * page to the dio instead.
 */
static int
submit_page_section(struct dio *dio, struct page *page,
                unsigned offset, unsigned len, sector_t blocknr)
{
        int ret = 0;

        /*
         * Can we just grow the current page's presence in the dio?
         */
        if (    (dio->cur_page == page) &&
                (dio->cur_page_offset + dio->cur_page_len == offset) &&
                (dio->cur_page_block +
                        (dio->cur_page_len >> dio->blkbits) == blocknr)) {
                dio->cur_page_len += len;

                /*
                 * If dio->boundary then we want to schedule the IO now to
                 * avoid metadata seeks.
                 */
                if (dio->boundary) {
                        ret = dio_send_cur_page(dio);
                        page_cache_release(dio->cur_page);
                        dio->cur_page = NULL;
                }
                goto out;
        }

        /*
         * If there's a deferred page already there then send it.
         */
        if (dio->cur_page) {
                ret = dio_send_cur_page(dio);
                page_cache_release(dio->cur_page);
                dio->cur_page = NULL;
                if (ret)
                        goto out;
        }

        page_cache_get(page);           /* It is in dio */
        dio->cur_page = page;
        dio->cur_page_offset = offset;
        dio->cur_page_len = len;
        dio->cur_page_block = blocknr;
out:
        return ret;
}

/*
 * Clean any dirty buffers in the blockdev mapping which alias newly-created
 * file blocks.  Only called for S_ISREG files - blockdevs do not set
 * buffer_new
 */
static void clean_blockdev_aliases(struct dio *dio)
{
        unsigned i;

        for (i = 0; i < dio->blocks_available; i++) {
                unmap_underlying_metadata(dio->map_bh.b_bdev,
                                        dio->map_bh.b_blocknr + i);
        }
}

/*
 * If we are not writing the entire block and get_block() allocated
 * the block for us, we need to fill-in the unused portion of the
 * block with zeros. This happens only if user-buffer, fileoffset or
 * io length is not filesystem block-size multiple.
 *
 * `end' is zero if we're doing the start of the IO, 1 at the end of the
 * IO.
 */
static void dio_zero_block(struct dio *dio, int end)
{
        unsigned dio_blocks_per_fs_block;
        unsigned this_chunk_blocks;     /* In dio_blocks */
        unsigned this_chunk_bytes;
        struct page *page;

        dio->start_zero_done = 1;
        if (!dio->blkfactor || !buffer_new(&dio->map_bh))
                return;

        dio_blocks_per_fs_block = 1 << dio->blkfactor;
        this_chunk_blocks = dio->block_in_file & (dio_blocks_per_fs_block - 1);

        if (!this_chunk_blocks)
                return;

        /*
         * We need to zero out part of an fs block.  It is either at the
         * beginning or the end of the fs block.
         */
        if (end) 
                this_chunk_blocks = dio_blocks_per_fs_block - this_chunk_blocks;

        this_chunk_bytes = this_chunk_blocks << dio->blkbits;

        page = ZERO_PAGE(dio->curr_user_address);
        if (submit_page_section(dio, page, 0, this_chunk_bytes, 
                                dio->next_block_for_io))
                return;

        dio->next_block_for_io += this_chunk_blocks;
}

/*
 * Walk the user pages, and the file, mapping blocks to disk and generating
 * a sequence of (page,offset,len,block) mappings.  These mappings are injected
 * into submit_page_section(), which takes care of the next stage of submission
 *
 * Direct IO against a blockdev is different from a file.  Because we can
 * happily perform page-sized but 512-byte aligned IOs.  It is important that
 * blockdev IO be able to have fine alignment and large sizes.
 *
 * So what we do is to permit the ->get_blocks function to populate bh.b_size
 * with the size of IO which is permitted at this offset and this i_blkbits.
 *
 * For best results, the blockdev should be set up with 512-byte i_blkbits and
 * it should set b_size to PAGE_SIZE or more inside get_blocks().  This gives
 * fine alignment but still allows this function to work in PAGE_SIZE units.
 */
static int do_direct_IO(struct dio *dio)
{
        const unsigned blkbits = dio->blkbits;
        const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
        struct page *page;
        unsigned block_in_page;
        struct buffer_head *map_bh = &dio->map_bh;
        int ret = 0;

        /* The I/O can start at any block offset within the first page */
        block_in_page = dio->first_block_in_page;

        while (dio->block_in_file < dio->final_block_in_request) {
                page = dio_get_page(dio);
                if (IS_ERR(page)) {
                        ret = PTR_ERR(page);
                        goto out;
                }

                while (block_in_page < blocks_per_page) {
                        unsigned offset_in_page = block_in_page << blkbits;
                        unsigned this_chunk_bytes;      /* # of bytes mapped */
                        unsigned this_chunk_blocks;     /* # of blocks */
                        unsigned u;

                        if (dio->blocks_available == 0) {
                                /*
                                 * Need to go and map some more disk
                                 */
                                unsigned long blkmask;
                                unsigned long dio_remainder;

                                ret = get_more_blocks(dio);
                                if (ret) {
                                        page_cache_release(page);
                                        goto out;
                                }
                                if (!buffer_mapped(map_bh))
                                        goto do_holes;

                                dio->blocks_available =
                                                map_bh->b_size >> dio->blkbits;
                                dio->next_block_for_io =
                                        map_bh->b_blocknr << dio->blkfactor;
                                if (buffer_new(map_bh))
                                        clean_blockdev_aliases(dio);

                                if (!dio->blkfactor)
                                        goto do_holes;

                                blkmask = (1 << dio->blkfactor) - 1;
                                dio_remainder = (dio->block_in_file & blkmask);

                                /*
                                 * If we are at the start of IO and that IO
                                 * starts partway into a fs-block,
                                 * dio_remainder will be non-zero.  If the IO
                                 * is a read then we can simply advance the IO
                                 * cursor to the first block which is to be
                                 * read.  But if the IO is a write and the
                                 * block was newly allocated we cannot do that;
                                 * the start of the fs block must be zeroed out
                                 * on-disk
                                 */
                                if (!buffer_new(map_bh))
                                        dio->next_block_for_io += dio_remainder;
                                dio->blocks_available -= dio_remainder;
                        }
do_holes:
                        /* Handle holes */
                        if (!buffer_mapped(map_bh)) {
                                char *kaddr;

                                if (dio->block_in_file >=
                                        i_size_read(dio->inode)>>blkbits) {
                                        /* We hit eof */
                                        page_cache_release(page);
                                        goto out;
                                }
                                kaddr = kmap_atomic(page, KM_USER0);
                                memset(kaddr + (block_in_page << blkbits),
                                                0, 1 << blkbits);
                                flush_dcache_page(page);
                                kunmap_atomic(kaddr, KM_USER0);
                                dio->block_in_file++;
                                block_in_page++;
                                goto next_block;
                        }

                        /*
                         * If we're performing IO which has an alignment which
                         * is finer than the underlying fs, go check to see if
                         * we must zero out the start of this block.
                         */
                        if (unlikely(dio->blkfactor && !dio->start_zero_done))
                                dio_zero_block(dio, 0);

                        /*
                         * Work out, in this_chunk_blocks, how much disk we
                         * can add to this page
                         */
                        this_chunk_blocks = dio->blocks_available;
                        u = (PAGE_SIZE - offset_in_page) >> blkbits;
                        if (this_chunk_blocks > u)
                                this_chunk_blocks = u;
                        u = dio->final_block_in_request - dio->block_in_file;
                        if (this_chunk_blocks > u)
                                this_chunk_blocks = u;
                        this_chunk_bytes = this_chunk_blocks << blkbits;
                        BUG_ON(this_chunk_bytes == 0);

                        dio->boundary = buffer_boundary(map_bh);
                        ret = submit_page_section(dio, page, offset_in_page,
                                this_chunk_bytes, dio->next_block_for_io);
                        if (ret) {
                                page_cache_release(page);
                                goto out;
                        }
                        dio->next_block_for_io += this_chunk_blocks;

                        dio->block_in_file += this_chunk_blocks;
                        block_in_page += this_chunk_blocks;
                        dio->blocks_available -= this_chunk_blocks;
next_block:
                        if (dio->block_in_file > dio->final_block_in_request)
                                BUG();
                        if (dio->block_in_file == dio->final_block_in_request)
                                break;
                }

                /* Drop the ref which was taken in get_user_pages() */
                page_cache_release(page);
                block_in_page = 0;
        }
out:
        return ret;
}

static int
direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode, 
        const struct iovec *iov, loff_t offset, unsigned long nr_segs, 
        unsigned blkbits, get_blocks_t get_blocks)
{
        unsigned long user_addr; 
        int seg;
        int ret = 0;
        int ret2;
        struct dio *dio;
        size_t bytes;

        dio = kmalloc(sizeof(*dio), GFP_KERNEL);
        if (!dio)
                return -ENOMEM;
        dio->is_async = !is_sync_kiocb(iocb);

        dio->bio = NULL;
        dio->inode = inode;
        dio->rw = rw;
        dio->blkbits = blkbits;
        dio->blkfactor = inode->i_blkbits - blkbits;
        dio->start_zero_done = 0;
        dio->block_in_file = offset >> blkbits;
        dio->blocks_available = 0;

        dio->cur_page = NULL;

        dio->boundary = 0;
        dio->reap_counter = 0;
        dio->get_blocks = get_blocks;
        dio->final_block_in_bio = -1;
        dio->next_block_for_io = -1;

        dio->page_errors = 0;
        dio->result = 0;
        dio->iocb = iocb;

        /*
         * BIO completion state.
         *
         * ->bio_count starts out at one, and we decrement it to zero after all
         * BIOs are submitted.  This to avoid the situation where a really fast
         * (or synchronous) device could take the count to zero while we're
         * still submitting BIOs.
         */
        atomic_set(&dio->bio_count, 1);
        atomic_set(&dio->bios_in_flight, 0);
        spin_lock_init(&dio->bio_list_lock);
        dio->bio_list = NULL;
        dio->waiter = NULL;

        dio->pages_in_io = 0;
        for (seg = 0; seg < nr_segs; seg++) 
                dio->pages_in_io += (iov[seg].iov_len >> blkbits) + 2; 

        for (seg = 0; seg < nr_segs; seg++) {
                user_addr = (unsigned long)iov[seg].iov_base;
                bytes = iov[seg].iov_len;

                /* Index into the first page of the first block */
                dio->first_block_in_page = (user_addr & ~PAGE_MASK) >> blkbits;
                dio->final_block_in_request = dio->block_in_file +
                                                (bytes >> blkbits);
                /* Page fetching state */
                dio->head = 0;
                dio->tail = 0;
                dio->curr_page = 0;

                dio->total_pages = 0;
                if (user_addr & (PAGE_SIZE-1)) {
                        dio->total_pages++;
                        bytes -= PAGE_SIZE - (user_addr & (PAGE_SIZE - 1));
                }
                dio->total_pages += (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
                dio->curr_user_address = user_addr;
        
                ret = do_direct_IO(dio);

                dio->result += iov[seg].iov_len -
                        ((dio->final_block_in_request - dio->block_in_file) <<
                                        blkbits);

                if (ret) {
                        dio_cleanup(dio);
                        break;
                }
        } /* end iovec loop */

        /*
         * There may be some unwritten disk at the end of a part-written
         * fs-block-sized block.  Go zero that now.
         */
        dio_zero_block(dio, 1);

        if (dio->cur_page) {
                ret2 = dio_send_cur_page(dio);
                if (ret == 0)
                        ret = ret2;
                page_cache_release(dio->cur_page);
                dio->cur_page = NULL;
        }
        if (dio->bio)
                dio_bio_submit(dio);

        /*
         * OK, all BIOs are submitted, so we can decrement bio_count to truly
         * reflect the number of to-be-processed BIOs.
         */
        if (dio->is_async) {
                if (ret == 0)
                        ret = dio->result;      /* Bytes written */
                finished_one_bio(dio);          /* This can free the dio */
                blk_run_queues();
        } else {
                finished_one_bio(dio);
                ret2 = dio_await_completion(dio);
                if (ret == 0)
                        ret = ret2;
                if (ret == 0)
                        ret = dio->page_errors;
                if (ret == 0 && dio->result) {
                        loff_t i_size = i_size_read(inode);

                        ret = dio->result;
                        /*
                         * Adjust the return value if the read crossed a
                         * non-block-aligned EOF.
                         */
                        if (rw == READ && (offset + ret > i_size))
                                ret = i_size - offset;
                }
                kfree(dio);
        }
        return ret;
}

/*
 * This is a library function for use by filesystem drivers.
 */
int
blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode, 
        struct block_device *bdev, const struct iovec *iov, loff_t offset, 
        unsigned long nr_segs, get_blocks_t get_blocks)
{
        int seg;
        size_t size;
        unsigned long addr;
        unsigned blkbits = inode->i_blkbits;
        unsigned bdev_blkbits = 0;
        unsigned blocksize_mask = (1 << blkbits) - 1;
        ssize_t retval = -EINVAL;

        if (bdev)
                bdev_blkbits = blksize_bits(bdev_hardsect_size(bdev));

        if (offset & blocksize_mask) {
                if (bdev)
                         blkbits = bdev_blkbits;
                blocksize_mask = (1 << blkbits) - 1;
                if (offset & blocksize_mask)
                        goto out;
        }

        /* Check the memory alignment.  Blocks cannot straddle pages */
        for (seg = 0; seg < nr_segs; seg++) {
                addr = (unsigned long)iov[seg].iov_base;
                size = iov[seg].iov_len;
                if ((addr & blocksize_mask) || (size & blocksize_mask))  {
                        if (bdev)
                                 blkbits = bdev_blkbits;
                        blocksize_mask = (1 << blkbits) - 1;
                        if ((addr & blocksize_mask) || (size & blocksize_mask))  
                                goto out;
                }
        }

        retval = direct_io_worker(rw, iocb, inode, iov, offset, 
                                nr_segs, blkbits, get_blocks);
out:
        return retval;
}