direct-io.c

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/*
 * fs/direct-io.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * O_DIRECT
 *
 * 04Jul2002    Andrew Morton
 *      Initial version
 * 11Sep2002    [email protected]
 *      added readv/writev support.
 * 29Oct2002    Andrew Morton
 *      rewrote bio_add_page() support.
 * 30Oct2002    [email protected]
 *      added support for non-aligned IO.
 * 06Nov2002    [email protected]
 *      added asynchronous IO support.
 * 21Jul2003    [email protected]
 *      added IO completion notifier.
 */

#include <linux/kernel.h>
#include <linux/module.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/task_io_accounting_ops.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 <linux/atomic.h>
#include <linux/prefetch.h>

/*
 * How many user pages to map in one call to get_user_pages().  This determines
 * the size of a structure in the slab cache
 */
#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.
 */

/* dio_state only used in the submission path */

struct dio_submit {
    struct bio *bio;        /* bio under assembly */
    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 */
    size_t  size;           /* total request size (doesn't change)*/
    sector_t block_in_file;     /* Current offset into the underlying
                       file in dio_block units. */
    unsigned blocks_available;  /* At block_in_file.  changes */
    int reap_counter;       /* rate limit reaping */
    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 */
    get_block_t *get_block;     /* block mapping function */
    dio_submit_t *submit_io;    /* IO submition function */

    loff_t logical_offset_in_bio;   /* current first logical block in bio */
    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 */

    /*
     * 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 */
    loff_t cur_page_fs_offset;  /* Offset in file */

    /*
     * 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().
     */
    unsigned head;          /* next page to process */
    unsigned tail;          /* last valid page + 1 */
};

/* dio_state communicated between submission path and end_io */
struct dio {
    int flags;          /* doesn't change */
    int rw;
    struct inode *inode;
    loff_t i_size;          /* i_size when submitted */
    dio_iodone_t *end_io;       /* IO completion function */

    void *private;          /* copy from map_bh.b_private */

    /* BIO completion state */
    spinlock_t bio_lock;        /* protects BIO fields below */
    int page_errors;        /* errno from get_user_pages() */
    int is_async;           /* is IO async ? */
    int io_error;           /* IO error in completion path */
    unsigned long refcount;     /* direct_io_worker() and bios */
    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 */
    ssize_t result;                 /* IO result */

    /*
     * pages[] (and any fields placed after it) are not zeroed out at
     * allocation time.  Don't add new fields after pages[] unless you
     * wish that they not be zeroed.
     */
    struct page *pages[DIO_PAGES];  /* page buffer */
} ____cacheline_aligned_in_smp;

static struct kmem_cache *dio_cache __read_mostly;

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

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

    nr_pages = min(sdio->total_pages - sdio->curr_page, DIO_PAGES);
    ret = get_user_pages_fast(
        sdio->curr_user_address,        /* Where from? */
        nr_pages,           /* How many pages? */
        dio->rw == READ,        /* Write to memory? */
        &dio->pages[0]);        /* Put results here */

    if (ret < 0 && sdio->blocks_available && (dio->rw & WRITE)) {
        struct page *page = ZERO_PAGE(0);
        /*
         * 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;
        page_cache_get(page);
        dio->pages[0] = page;
        sdio->head = 0;
        sdio->tail = 1;
        ret = 0;
        goto out;
    }

    if (ret >= 0) {
        sdio->curr_user_address += ret * PAGE_SIZE;
        sdio->curr_page += ret;
        sdio->head = 0;
        sdio->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 inline struct page *dio_get_page(struct dio *dio,
        struct dio_submit *sdio)
{
    if (dio_pages_present(sdio) == 0) {
        int ret;

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

static ssize_t dio_complete(struct dio *dio, loff_t offset, ssize_t ret, bool is_async)
{
    ssize_t transferred = 0;

    /*
     * AIO submission can race with bio completion to get here while
     * expecting to have the last io completed by bio completion.
     * In that case -EIOCBQUEUED is in fact not an error we want
     * to preserve through this call.
     */
    if (ret == -EIOCBQUEUED)
        ret = 0;

    if (dio->result) {
        transferred = dio->result;

        /* Check for short read case */
        if ((dio->rw == READ) && ((offset + transferred) > dio->i_size))
            transferred = dio->i_size - offset;
    }

    if (ret == 0)
        ret = dio->page_errors;
    if (ret == 0)
        ret = dio->io_error;
    if (ret == 0)
        ret = transferred;

    if (dio->end_io && dio->result) {
        dio->end_io(dio->iocb, offset, transferred,
                dio->private, ret, is_async);
    } else {
        if (is_async)
            aio_complete(dio->iocb, ret, 0);
        inode_dio_done(dio->inode);
    }

    return ret;
}

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

    /* cleanup the bio */
    dio_bio_complete(dio, bio);

    spin_lock_irqsave(&dio->bio_lock, flags);
    remaining = --dio->refcount;
    if (remaining == 1 && dio->waiter)
        wake_up_process(dio->waiter);
    spin_unlock_irqrestore(&dio->bio_lock, flags);

    if (remaining == 0) {
        dio_complete(dio, dio->iocb->ki_pos, 0, true);
        kmem_cache_free(dio_cache, dio);
    }
}

/*
 * 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 void dio_bio_end_io(struct bio *bio, int error)
{
    struct dio *dio = bio->bi_private;
    unsigned long flags;

    spin_lock_irqsave(&dio->bio_lock, flags);
    bio->bi_private = dio->bio_list;
    dio->bio_list = bio;
    if (--dio->refcount == 1 && dio->waiter)
        wake_up_process(dio->waiter);
    spin_unlock_irqrestore(&dio->bio_lock, flags);
}

void dio_end_io(struct bio *bio, int error)
{
    struct dio *dio = bio->bi_private;

    if (dio->is_async)
        dio_bio_end_aio(bio, error);
    else
        dio_bio_end_io(bio, error);
}
EXPORT_SYMBOL_GPL(dio_end_io);

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

    /*
     * bio_alloc() is guaranteed to return a bio when called with
     * __GFP_WAIT and we request a valid number of vectors.
     */
    bio = bio_alloc(GFP_KERNEL, nr_vecs);

    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;

    sdio->bio = bio;
    sdio->logical_offset_in_bio = sdio->cur_page_fs_offset;
}

/*
 * 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().
 *
 * bios hold a dio reference between submit_bio and ->end_io.
 */
static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
{
    struct bio *bio = sdio->bio;
    unsigned long flags;

    bio->bi_private = dio;

    spin_lock_irqsave(&dio->bio_lock, flags);
    dio->refcount++;
    spin_unlock_irqrestore(&dio->bio_lock, flags);

    if (dio->is_async && dio->rw == READ)
        bio_set_pages_dirty(bio);

    if (sdio->submit_io)
        sdio->submit_io(dio->rw, bio, dio->inode,
                   sdio->logical_offset_in_bio);
    else
        submit_bio(dio->rw, bio);

    sdio->bio = NULL;
    sdio->boundary = 0;
    sdio->logical_offset_in_bio = 0;
}

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

/*
 * Wait for the next BIO to complete.  Remove it and return it.  NULL is
 * returned once all BIOs have been completed.  This must only be called once
 * all bios have been issued so that dio->refcount can only decrease.  This
 * requires that that the caller hold a reference on the dio.
 */
static struct bio *dio_await_one(struct dio *dio)
{
    unsigned long flags;
    struct bio *bio = NULL;

    spin_lock_irqsave(&dio->bio_lock, flags);

    /*
     * Wait as long as the list is empty and there are bios in flight.  bio
     * completion drops the count, maybe adds to the list, and wakes while
     * holding the bio_lock so we don't need set_current_state()'s barrier
     * and can call it after testing our condition.
     */
    while (dio->refcount > 1 && dio->bio_list == NULL) {
        __set_current_state(TASK_UNINTERRUPTIBLE);
        dio->waiter = current;
        spin_unlock_irqrestore(&dio->bio_lock, flags);
        io_schedule();
        /* wake up sets us TASK_RUNNING */
        spin_lock_irqsave(&dio->bio_lock, flags);
        dio->waiter = NULL;
    }
    if (dio->bio_list) {
        bio = dio->bio_list;
        dio->bio_list = bio->bi_private;
    }
    spin_unlock_irqrestore(&dio->bio_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->io_error = -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 && !PageCompound(page))
                set_page_dirty_lock(page);
            page_cache_release(page);
        }
        bio_put(bio);
    }
    return uptodate ? 0 : -EIO;
}

/*
 * Wait on and process all in-flight BIOs.  This must only be called once
 * all bios have been issued so that the refcount can only decrease.
 * This just waits for all bios to make it through dio_bio_complete.  IO
 * errors are propagated through dio->io_error and should be propagated via
 * dio_complete().
 */
static void dio_await_completion(struct dio *dio)
{
    struct bio *bio;
    do {
        bio = dio_await_one(dio);
        if (bio)
            dio_bio_complete(dio, bio);
    } while (bio);
}

/*
 * 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 inline int dio_bio_reap(struct dio *dio, struct dio_submit *sdio)
{
    int ret = 0;

    if (sdio->reap_counter++ >= 64) {
        while (dio->bio_list) {
            unsigned long flags;
            struct bio *bio;
            int ret2;

            spin_lock_irqsave(&dio->bio_lock, flags);
            bio = dio->bio_list;
            dio->bio_list = bio->bi_private;
            spin_unlock_irqrestore(&dio->bio_lock, flags);
            ret2 = dio_bio_complete(dio, bio);
            if (ret == 0)
                ret = ret2;
        }
        sdio->reap_counter = 0;
    }
    return ret;
}

/*
 * Call into the fs to map some more disk blocks.  We record the current number
 * of available blocks at sdio->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_block() 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_block() as it walks the hole.
 */
static int get_more_blocks(struct dio *dio, struct dio_submit *sdio,
               struct buffer_head *map_bh)
{
    int ret;
    sector_t fs_startblk;   /* Into file, in filesystem-sized blocks */
    sector_t fs_endblk; /* Into file, in filesystem-sized blocks */
    unsigned long fs_count; /* Number of filesystem-sized blocks */
    int create;
    unsigned int i_blkbits = sdio->blkbits + sdio->blkfactor;

    /*
     * 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) {
        BUG_ON(sdio->block_in_file >= sdio->final_block_in_request);
        fs_startblk = sdio->block_in_file >> sdio->blkfactor;
        fs_endblk = (sdio->final_block_in_request - 1) >>
                    sdio->blkfactor;
        fs_count = fs_endblk - fs_startblk + 1;

        map_bh->b_state = 0;
        map_bh->b_size = fs_count << i_blkbits;

        /*
         * For writes inside i_size on a DIO_SKIP_HOLES filesystem we
         * forbid block creations: only overwrites are permitted.
         * We will return early to the caller once we see an
         * unmapped buffer head returned, and the caller will fall
         * back to buffered I/O.
         *
         * Otherwise the decision is left to the get_blocks method,
         * which may decide to handle it or also return an unmapped
         * buffer head.
         */
        create = dio->rw & WRITE;
        if (dio->flags & DIO_SKIP_HOLES) {
            if (sdio->block_in_file < (i_size_read(dio->inode) >>
                            sdio->blkbits))
                create = 0;
        }

        ret = (*sdio->get_block)(dio->inode, fs_startblk,
                        map_bh, create);

        /* Store for completion */
        dio->private = map_bh->b_private;
    }
    return ret;
}

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

    ret = dio_bio_reap(dio, sdio);
    if (ret)
        goto out;
    sector = start_sector << (sdio->blkbits - 9);
    nr_pages = min(sdio->pages_in_io, bio_get_nr_vecs(map_bh->b_bdev));
    nr_pages = min(nr_pages, BIO_MAX_PAGES);
    BUG_ON(nr_pages <= 0);
    dio_bio_alloc(dio, sdio, map_bh->b_bdev, sector, nr_pages);
    sdio->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 inline int dio_bio_add_page(struct dio_submit *sdio)
{
    int ret;

    ret = bio_add_page(sdio->bio, sdio->cur_page,
            sdio->cur_page_len, sdio->cur_page_offset);
    if (ret == sdio->cur_page_len) {
        /*
         * Decrement count only, if we are done with this page
         */
        if ((sdio->cur_page_len + sdio->cur_page_offset) == PAGE_SIZE)
            sdio->pages_in_io--;
        page_cache_get(sdio->cur_page);
        sdio->final_block_in_bio = sdio->cur_page_block +
            (sdio->cur_page_len >> sdio->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 inline int dio_send_cur_page(struct dio *dio, struct dio_submit *sdio,
        struct buffer_head *map_bh)
{
    int ret = 0;

    if (sdio->bio) {
        loff_t cur_offset = sdio->cur_page_fs_offset;
        loff_t bio_next_offset = sdio->logical_offset_in_bio +
            sdio->bio->bi_size;

        /*
         * See whether this new request is contiguous with the old.
         *
         * Btrfs cannot handle having logically non-contiguous requests
         * submitted.  For example if you have
         *
         * Logical:  [0-4095][HOLE][8192-12287]
         * Physical: [0-4095]      [4096-8191]
         *
         * We cannot submit those pages together as one BIO.  So if our
         * current logical offset in the file does not equal what would
         * be the next logical offset in the bio, submit the bio we
         * have.
         */
        if (sdio->final_block_in_bio != sdio->cur_page_block ||
            cur_offset != bio_next_offset)
            dio_bio_submit(dio, sdio);
        /*
         * Submit now if the underlying fs is about to perform a
         * metadata read
         */
        else if (sdio->boundary)
            dio_bio_submit(dio, sdio);
    }

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

    if (dio_bio_add_page(sdio) != 0) {
        dio_bio_submit(dio, sdio);
        ret = dio_new_bio(dio, sdio, sdio->cur_page_block, map_bh);
        if (ret == 0) {
            ret = dio_bio_add_page(sdio);
            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 inline int
submit_page_section(struct dio *dio, struct dio_submit *sdio, struct page *page,
            unsigned offset, unsigned len, sector_t blocknr,
            struct buffer_head *map_bh)
{
    int ret = 0;

    if (dio->rw & WRITE) {
        /*
         * Read accounting is performed in submit_bio()
         */
        task_io_account_write(len);
    }

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

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

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

    page_cache_get(page);       /* It is in dio */
    sdio->cur_page = page;
    sdio->cur_page_offset = offset;
    sdio->cur_page_len = len;
    sdio->cur_page_block = blocknr;
    sdio->cur_page_fs_offset = sdio->block_in_file << sdio->blkbits;
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, struct buffer_head *map_bh)
{
    unsigned i;
    unsigned nblocks;

    nblocks = map_bh->b_size >> dio->inode->i_blkbits;

    for (i = 0; i < nblocks; i++) {
        unmap_underlying_metadata(map_bh->b_bdev,
                      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 inline void dio_zero_block(struct dio *dio, struct dio_submit *sdio,
        int end, struct buffer_head *map_bh)
{
    unsigned dio_blocks_per_fs_block;
    unsigned this_chunk_blocks; /* In dio_blocks */
    unsigned this_chunk_bytes;
    struct page *page;

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

    dio_blocks_per_fs_block = 1 << sdio->blkfactor;
    this_chunk_blocks = sdio->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 << sdio->blkbits;

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

    sdio->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_block 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_block().  This gives
 * fine alignment but still allows this function to work in PAGE_SIZE units.
 */
static int do_direct_IO(struct dio *dio, struct dio_submit *sdio,
            struct buffer_head *map_bh)
{
    const unsigned blkbits = sdio->blkbits;
    const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
    struct page *page;
    unsigned block_in_page;
    int ret = 0;

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

    while (sdio->block_in_file < sdio->final_block_in_request) {
        page = dio_get_page(dio, sdio);
        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 (sdio->blocks_available == 0) {
                /*
                 * Need to go and map some more disk
                 */
                unsigned long blkmask;
                unsigned long dio_remainder;

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

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

                if (!sdio->blkfactor)
                    goto do_holes;

                blkmask = (1 << sdio->blkfactor) - 1;
                dio_remainder = (sdio->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))
                    sdio->next_block_for_io += dio_remainder;
                sdio->blocks_available -= dio_remainder;
            }
do_holes:
            /* Handle holes */
            if (!buffer_mapped(map_bh)) {
                loff_t i_size_aligned;

                /* AKPM: eargh, -ENOTBLK is a hack */
                if (dio->rw & WRITE) {
                    page_cache_release(page);
                    return -ENOTBLK;
                }

                /*
                 * Be sure to account for a partial block as the
                 * last block in the file
                 */
                i_size_aligned = ALIGN(i_size_read(dio->inode),
                            1 << blkbits);
                if (sdio->block_in_file >=
                        i_size_aligned >> blkbits) {
                    /* We hit eof */
                    page_cache_release(page);
                    goto out;
                }
                zero_user(page, block_in_page << blkbits,
                        1 << blkbits);
                sdio->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(sdio->blkfactor && !sdio->start_zero_done))
                dio_zero_block(dio, sdio, 0, map_bh);

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

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

            sdio->block_in_file += this_chunk_blocks;
            block_in_page += this_chunk_blocks;
            sdio->blocks_available -= this_chunk_blocks;
next_block:
            BUG_ON(sdio->block_in_file > sdio->final_block_in_request);
            if (sdio->block_in_file == sdio->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 inline int drop_refcount(struct dio *dio)
{
    int ret2;
    unsigned long flags;

    /*
     * Sync will always be dropping the final ref and completing the
     * operation.  AIO can if it was a broken operation described above or
     * in fact if all the bios race to complete before we get here.  In
     * that case dio_complete() translates the EIOCBQUEUED into the proper
     * return code that the caller will hand to aio_complete().
     *
     * This is managed by the bio_lock instead of being an atomic_t so that
     * completion paths can drop their ref and use the remaining count to
     * decide to wake the submission path atomically.
     */
    spin_lock_irqsave(&dio->bio_lock, flags);
    ret2 = --dio->refcount;
    spin_unlock_irqrestore(&dio->bio_lock, flags);
    return ret2;
}

/*
 * This is a library function for use by filesystem drivers.
 *
 * The locking rules are governed by the flags parameter:
 *  - if the flags value contains DIO_LOCKING we use a fancy locking
 *    scheme for dumb filesystems.
 *    For writes this function is called under i_mutex and returns with
 *    i_mutex held, for reads, i_mutex is not held on entry, but it is
 *    taken and dropped again before returning.
 *  - if the flags value does NOT contain DIO_LOCKING we don't use any
 *    internal locking but rather rely on the filesystem to synchronize
 *    direct I/O reads/writes versus each other and truncate.
 *
 * To help with locking against truncate we incremented the i_dio_count
 * counter before starting direct I/O, and decrement it once we are done.
 * Truncate can wait for it to reach zero to provide exclusion.  It is
 * expected that filesystem provide exclusion between new direct I/O
 * and truncates.  For DIO_LOCKING filesystems this is done by i_mutex,
 * but other filesystems need to take care of this on their own.
 *
 * NOTE: if you pass "sdio" to anything by pointer make sure that function
 * is always inlined. Otherwise gcc is unable to split the structure into
 * individual fields and will generate much worse code. This is important
 * for the whole file.
 */
static inline ssize_t
do_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_block_t get_block, dio_iodone_t end_io,
    dio_submit_t submit_io, int flags)
{
    int seg;
    size_t size;
    unsigned long addr;
    unsigned i_blkbits = ACCESS_ONCE(inode->i_blkbits);
    unsigned blkbits = i_blkbits;
    unsigned blocksize_mask = (1 << blkbits) - 1;
    ssize_t retval = -EINVAL;
    loff_t end = offset;
    struct dio *dio;
    struct dio_submit sdio = { 0, };
    unsigned long user_addr;
    size_t bytes;
    struct buffer_head map_bh = { 0, };
    struct blk_plug plug;

    if (rw & WRITE)
        rw = WRITE_ODIRECT;

    /*
     * Avoid references to bdev if not absolutely needed to give
     * the early prefetch in the caller enough time.
     */

    if (offset & blocksize_mask) {
        if (bdev)
            blkbits = blksize_bits(bdev_logical_block_size(bdev));
        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;
        end += size;
        if (unlikely((addr & blocksize_mask) ||
                 (size & blocksize_mask))) {
            if (bdev)
                blkbits = blksize_bits(
                     bdev_logical_block_size(bdev));
            blocksize_mask = (1 << blkbits) - 1;
            if ((addr & blocksize_mask) || (size & blocksize_mask))
                goto out;
        }
    }

    /* watch out for a 0 len io from a tricksy fs */
    if (rw == READ && end == offset)
        return 0;

    dio = kmem_cache_alloc(dio_cache, GFP_KERNEL);
    retval = -ENOMEM;
    if (!dio)
        goto out;
    /*
     * Believe it or not, zeroing out the page array caused a .5%
     * performance regression in a database benchmark.  So, we take
     * care to only zero out what's needed.
     */
    memset(dio, 0, offsetof(struct dio, pages));

    dio->flags = flags;
    if (dio->flags & DIO_LOCKING) {
        if (rw == READ) {
            struct address_space *mapping =
                    iocb->ki_filp->f_mapping;

            /* will be released by direct_io_worker */
            mutex_lock(&inode->i_mutex);

            retval = filemap_write_and_wait_range(mapping, offset,
                                  end - 1);
            if (retval) {
                mutex_unlock(&inode->i_mutex);
                kmem_cache_free(dio_cache, dio);
                goto out;
            }
        }
    }

    /*
     * Will be decremented at I/O completion time.
     */
    atomic_inc(&inode->i_dio_count);

    /*
     * For file extending writes updating i_size before data
     * writeouts complete can expose uninitialized blocks. So
     * even for AIO, we need to wait for i/o to complete before
     * returning in this case.
     */
    dio->is_async = !is_sync_kiocb(iocb) && !((rw & WRITE) &&
        (end > i_size_read(inode)));

    retval = 0;

    dio->inode = inode;
    dio->rw = rw;
    sdio.blkbits = blkbits;
    sdio.blkfactor = i_blkbits - blkbits;
    sdio.block_in_file = offset >> blkbits;

    sdio.get_block = get_block;
    dio->end_io = end_io;
    sdio.submit_io = submit_io;
    sdio.final_block_in_bio = -1;
    sdio.next_block_for_io = -1;

    dio->iocb = iocb;
    dio->i_size = i_size_read(inode);

    spin_lock_init(&dio->bio_lock);
    dio->refcount = 1;

    /*
     * In case of non-aligned buffers, we may need 2 more
     * pages since we need to zero out first and last block.
     */
    if (unlikely(sdio.blkfactor))
        sdio.pages_in_io = 2;

    for (seg = 0; seg < nr_segs; seg++) {
        user_addr = (unsigned long)iov[seg].iov_base;
        sdio.pages_in_io +=
            ((user_addr + iov[seg].iov_len + PAGE_SIZE-1) /
                PAGE_SIZE - user_addr / PAGE_SIZE);
    }

    blk_start_plug(&plug);

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

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

        sdio.total_pages = 0;
        if (user_addr & (PAGE_SIZE-1)) {
            sdio.total_pages++;
            bytes -= PAGE_SIZE - (user_addr & (PAGE_SIZE - 1));
        }
        sdio.total_pages += (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
        sdio.curr_user_address = user_addr;

        retval = do_direct_IO(dio, &sdio, &map_bh);

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

        if (retval) {
            dio_cleanup(dio, &sdio);
            break;
        }
    } /* end iovec loop */

    if (retval == -ENOTBLK) {
        /*
         * The remaining part of the request will be
         * be handled by buffered I/O when we return
         */
        retval = 0;
    }
    /*
     * 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, &sdio, 1, &map_bh);

    if (sdio.cur_page) {
        ssize_t ret2;

        ret2 = dio_send_cur_page(dio, &sdio, &map_bh);
        if (retval == 0)
            retval = ret2;
        page_cache_release(sdio.cur_page);
        sdio.cur_page = NULL;
    }
    if (sdio.bio)
        dio_bio_submit(dio, &sdio);

    blk_finish_plug(&plug);

    /*
     * It is possible that, we return short IO due to end of file.
     * In that case, we need to release all the pages we got hold on.
     */
    dio_cleanup(dio, &sdio);

    /*
     * All block lookups have been performed. For READ requests
     * we can let i_mutex go now that its achieved its purpose
     * of protecting us from looking up uninitialized blocks.
     */
    if (rw == READ && (dio->flags & DIO_LOCKING))
        mutex_unlock(&dio->inode->i_mutex);

    /*
     * The only time we want to leave bios in flight is when a successful
     * partial aio read or full aio write have been setup.  In that case
     * bio completion will call aio_complete.  The only time it's safe to
     * call aio_complete is when we return -EIOCBQUEUED, so we key on that.
     * This had *better* be the only place that raises -EIOCBQUEUED.
     */
    BUG_ON(retval == -EIOCBQUEUED);
    if (dio->is_async && retval == 0 && dio->result &&
        ((rw == READ) || (dio->result == sdio.size)))
        retval = -EIOCBQUEUED;

    if (retval != -EIOCBQUEUED)
        dio_await_completion(dio);

    if (drop_refcount(dio) == 0) {
        retval = dio_complete(dio, offset, retval, false);
        kmem_cache_free(dio_cache, dio);
    } else
        BUG_ON(retval != -EIOCBQUEUED);

out:
    return retval;
}

ssize_t
__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_block_t get_block, dio_iodone_t end_io,
    dio_submit_t submit_io, int flags)
{
    /*
     * The block device state is needed in the end to finally
     * submit everything.  Since it's likely to be cache cold
     * prefetch it here as first thing to hide some of the
     * latency.
     *
     * Attempt to prefetch the pieces we likely need later.
     */
    prefetch(&bdev->bd_disk->part_tbl);
    prefetch(bdev->bd_queue);
    prefetch((char *)bdev->bd_queue + SMP_CACHE_BYTES);

    return do_blockdev_direct_IO(rw, iocb, inode, bdev, iov, offset,
                     nr_segs, get_block, end_io,
                     submit_io, flags);
}

EXPORT_SYMBOL(__blockdev_direct_IO);

static __init int dio_init(void)
{
    dio_cache = KMEM_CACHE(dio, SLAB_PANIC);
    return 0;
}
module_init(dio_init)