direct-io: Implement generic deferred AIO completions

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

/*
 * fs/direct-io.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * O_DIRECT
 *
 * 04Jul2002    Andrew Morton
 *              Initial version
 * 11Sep2002    janetinc@us.ibm.com
 *              added readv/writev support.
 * 29Oct2002    Andrew Morton
 *              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.
 * 21Jul2003    nathans@sgi.com
 *              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>
#include <linux/aio.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 ? */
        bool defer_completion;          /* defer AIO completion to workqueue? */
        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.
         */
        union {
                struct page *pages[DIO_PAGES];  /* page buffer */
                struct work_struct complete_work;/* deferred AIO completion */
        };
} ____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++];
}

/**
 * dio_complete() - called when all DIO BIO I/O has been completed
 * @offset: the byte offset in the file of the completed operation
 *
 * This drops i_dio_count, lets interested parties know that a DIO operation
 * has completed, and calculates the resulting return code for the operation.
 *
 * It lets the filesystem know if it registered an interest earlier via
 * get_block.  Pass the private field of the map buffer_head so that
 * filesystems can use it to hold additional state between get_block calls and
 * dio_complete.
 */
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);

        inode_dio_done(dio->inode);
        if (is_async)
                aio_complete(dio->iocb, ret, 0);

        kmem_cache_free(dio_cache, dio);
        return ret;
}

static void dio_aio_complete_work(struct work_struct *work)
{
        struct dio *dio = container_of(work, struct dio, complete_work);

        dio_complete(dio, dio->iocb->ki_pos, 0, true);
}

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) {
                if (dio->result && dio->defer_completion) {
                        INIT_WORK(&dio->complete_work, dio_aio_complete_work);
                        queue_work(dio->inode->i_sb->s_dio_done_wq,
                                   &dio->complete_work);
                } else {
                        dio_complete(dio, dio->iocb->ki_pos, 0, true);
                }
        }
}

/*
 * 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);
}

/**
 * dio_end_io - handle the end io action for the given bio
 * @bio: The direct io bio thats being completed
 * @error: Error if there was one
 *
 * This is meant to be called by any filesystem that uses their own dio_submit_t
 * so that the DIO specific endio actions are dealt with after the filesystem
 * has done it's completion work.
 */
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;
        unsigned i;

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

        if (dio->is_async && dio->rw == READ) {
                bio_check_pages_dirty(bio);     /* transfers ownership */
        } else {
                bio_for_each_segment_all(bvec, bio, i) {
                        struct page *page = bvec->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;
}

/*
 * Create workqueue for deferred direct IO completions. We allocate the
 * workqueue when it's first needed. This avoids creating workqueue for
 * filesystems that don't need it and also allows us to create the workqueue
 * late enough so the we can include s_id in the name of the workqueue.
 */
static int sb_init_dio_done_wq(struct super_block *sb)
{
        struct workqueue_struct *wq = alloc_workqueue("dio/%s",
                                                      WQ_MEM_RECLAIM, 0,
                                                      sb->s_id);
        if (!wq)
                return -ENOMEM;
        /*
         * This has to be atomic as more DIOs can race to create the workqueue
         */
        cmpxchg(&sb->s_dio_done_wq, NULL, wq);
        /* Someone created workqueue before us? Free ours... */
        if (wq != sb->s_dio_done_wq)
                destroy_workqueue(wq);
        return 0;
}

static int dio_set_defer_completion(struct dio *dio)
{
        struct super_block *sb = dio->inode->i_sb;

        if (dio->defer_completion)
                return 0;
        dio->defer_completion = true;
        if (!sb->s_dio_done_wq)
                return sb_init_dio_done_wq(sb);
        return 0;
}

/*
 * 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;

                if (ret == 0 && buffer_defer_completion(map_bh))
                        ret = dio_set_defer_completion(dio);
        }
        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);
        }

        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;
                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)
                        return ret;
        }

        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:
        /*
         * 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);
                dio_bio_submit(dio, sdio);
                page_cache_release(sdio->cur_page);
                sdio->cur_page = NULL;
        }
        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);

                        if (this_chunk_blocks == sdio->blocks_available)
                                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);
        } 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)