xfs_buf.c

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/*
 * Copyright (c) 2000-2006 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#include "xfs.h"
#include <linux/stddef.h>
#include <linux/errno.h>
#include <linux/gfp.h>
#include <linux/pagemap.h>
#include <linux/init.h>
#include <linux/vmalloc.h>
#include <linux/bio.h>
#include <linux/sysctl.h>
#include <linux/proc_fs.h>
#include <linux/workqueue.h>
#include <linux/percpu.h>
#include <linux/blkdev.h>
#include <linux/hash.h>
#include <linux/kthread.h>
#include <linux/migrate.h>
#include <linux/backing-dev.h>
#include <linux/freezer.h>

#include "xfs_sb.h"
#include "xfs_log.h"
#include "xfs_ag.h"
#include "xfs_mount.h"
#include "xfs_trace.h"

static kmem_zone_t *xfs_buf_zone;

static struct workqueue_struct *xfslogd_workqueue;

#ifdef XFS_BUF_LOCK_TRACKING
# define XB_SET_OWNER(bp)   ((bp)->b_last_holder = current->pid)
# define XB_CLEAR_OWNER(bp) ((bp)->b_last_holder = -1)
# define XB_GET_OWNER(bp)   ((bp)->b_last_holder)
#else
# define XB_SET_OWNER(bp)   do { } while (0)
# define XB_CLEAR_OWNER(bp) do { } while (0)
# define XB_GET_OWNER(bp)   do { } while (0)
#endif

#define xb_to_gfp(flags) \
    ((((flags) & XBF_READ_AHEAD) ? __GFP_NORETRY : GFP_NOFS) | __GFP_NOWARN)


static inline int
xfs_buf_is_vmapped(
    struct xfs_buf  *bp)
{
    /*
     * Return true if the buffer is vmapped.
     *
     * b_addr is null if the buffer is not mapped, but the code is clever
     * enough to know it doesn't have to map a single page, so the check has
     * to be both for b_addr and bp->b_page_count > 1.
     */
    return bp->b_addr && bp->b_page_count > 1;
}

static inline int
xfs_buf_vmap_len(
    struct xfs_buf  *bp)
{
    return (bp->b_page_count * PAGE_SIZE) - bp->b_offset;
}

/*
 * xfs_buf_lru_add - add a buffer to the LRU.
 *
 * The LRU takes a new reference to the buffer so that it will only be freed
 * once the shrinker takes the buffer off the LRU.
 */
STATIC void
xfs_buf_lru_add(
    struct xfs_buf  *bp)
{
    struct xfs_buftarg *btp = bp->b_target;

    spin_lock(&btp->bt_lru_lock);
    if (list_empty(&bp->b_lru)) {
        atomic_inc(&bp->b_hold);
        list_add_tail(&bp->b_lru, &btp->bt_lru);
        btp->bt_lru_nr++;
        bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
    }
    spin_unlock(&btp->bt_lru_lock);
}

/*
 * xfs_buf_lru_del - remove a buffer from the LRU
 *
 * The unlocked check is safe here because it only occurs when there are not
 * b_lru_ref counts left on the inode under the pag->pag_buf_lock. it is there
 * to optimise the shrinker removing the buffer from the LRU and calling
 * xfs_buf_free(). i.e. it removes an unnecessary round trip on the
 * bt_lru_lock.
 */
STATIC void
xfs_buf_lru_del(
    struct xfs_buf  *bp)
{
    struct xfs_buftarg *btp = bp->b_target;

    if (list_empty(&bp->b_lru))
        return;

    spin_lock(&btp->bt_lru_lock);
    if (!list_empty(&bp->b_lru)) {
        list_del_init(&bp->b_lru);
        btp->bt_lru_nr--;
    }
    spin_unlock(&btp->bt_lru_lock);
}

/*
 * When we mark a buffer stale, we remove the buffer from the LRU and clear the
 * b_lru_ref count so that the buffer is freed immediately when the buffer
 * reference count falls to zero. If the buffer is already on the LRU, we need
 * to remove the reference that LRU holds on the buffer.
 *
 * This prevents build-up of stale buffers on the LRU.
 */
void
xfs_buf_stale(
    struct xfs_buf  *bp)
{
    ASSERT(xfs_buf_islocked(bp));

    bp->b_flags |= XBF_STALE;

    /*
     * Clear the delwri status so that a delwri queue walker will not
     * flush this buffer to disk now that it is stale. The delwri queue has
     * a reference to the buffer, so this is safe to do.
     */
    bp->b_flags &= ~_XBF_DELWRI_Q;

    atomic_set(&(bp)->b_lru_ref, 0);
    if (!list_empty(&bp->b_lru)) {
        struct xfs_buftarg *btp = bp->b_target;

        spin_lock(&btp->bt_lru_lock);
        if (!list_empty(&bp->b_lru) &&
            !(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
            list_del_init(&bp->b_lru);
            btp->bt_lru_nr--;
            atomic_dec(&bp->b_hold);
        }
        spin_unlock(&btp->bt_lru_lock);
    }
    ASSERT(atomic_read(&bp->b_hold) >= 1);
}

static int
xfs_buf_get_maps(
    struct xfs_buf      *bp,
    int         map_count)
{
    ASSERT(bp->b_maps == NULL);
    bp->b_map_count = map_count;

    if (map_count == 1) {
        bp->b_maps = &bp->b_map;
        return 0;
    }

    bp->b_maps = kmem_zalloc(map_count * sizeof(struct xfs_buf_map),
                KM_NOFS);
    if (!bp->b_maps)
        return ENOMEM;
    return 0;
}

/*
 *  Frees b_pages if it was allocated.
 */
static void
xfs_buf_free_maps(
    struct xfs_buf  *bp)
{
    if (bp->b_maps != &bp->b_map) {
        kmem_free(bp->b_maps);
        bp->b_maps = NULL;
    }
}

struct xfs_buf *
_xfs_buf_alloc(
    struct xfs_buftarg  *target,
    struct xfs_buf_map  *map,
    int         nmaps,
    xfs_buf_flags_t     flags)
{
    struct xfs_buf      *bp;
    int         error;
    int         i;

    bp = kmem_zone_zalloc(xfs_buf_zone, KM_NOFS);
    if (unlikely(!bp))
        return NULL;

    /*
     * We don't want certain flags to appear in b_flags unless they are
     * specifically set by later operations on the buffer.
     */
    flags &= ~(XBF_UNMAPPED | XBF_TRYLOCK | XBF_ASYNC | XBF_READ_AHEAD);

    atomic_set(&bp->b_hold, 1);
    atomic_set(&bp->b_lru_ref, 1);
    init_completion(&bp->b_iowait);
    INIT_LIST_HEAD(&bp->b_lru);
    INIT_LIST_HEAD(&bp->b_list);
    RB_CLEAR_NODE(&bp->b_rbnode);
    sema_init(&bp->b_sema, 0); /* held, no waiters */
    XB_SET_OWNER(bp);
    bp->b_target = target;
    bp->b_flags = flags;

    /*
     * Set length and io_length to the same value initially.
     * I/O routines should use io_length, which will be the same in
     * most cases but may be reset (e.g. XFS recovery).
     */
    error = xfs_buf_get_maps(bp, nmaps);
    if (error)  {
        kmem_zone_free(xfs_buf_zone, bp);
        return NULL;
    }

    bp->b_bn = map[0].bm_bn;
    bp->b_length = 0;
    for (i = 0; i < nmaps; i++) {
        bp->b_maps[i].bm_bn = map[i].bm_bn;
        bp->b_maps[i].bm_len = map[i].bm_len;
        bp->b_length += map[i].bm_len;
    }
    bp->b_io_length = bp->b_length;

    atomic_set(&bp->b_pin_count, 0);
    init_waitqueue_head(&bp->b_waiters);

    XFS_STATS_INC(xb_create);
    trace_xfs_buf_init(bp, _RET_IP_);

    return bp;
}

/*
 *  Allocate a page array capable of holding a specified number
 *  of pages, and point the page buf at it.
 */
STATIC int
_xfs_buf_get_pages(
    xfs_buf_t       *bp,
    int         page_count,
    xfs_buf_flags_t     flags)
{
    /* Make sure that we have a page list */
    if (bp->b_pages == NULL) {
        bp->b_page_count = page_count;
        if (page_count <= XB_PAGES) {
            bp->b_pages = bp->b_page_array;
        } else {
            bp->b_pages = kmem_alloc(sizeof(struct page *) *
                         page_count, KM_NOFS);
            if (bp->b_pages == NULL)
                return -ENOMEM;
        }
        memset(bp->b_pages, 0, sizeof(struct page *) * page_count);
    }
    return 0;
}

/*
 *  Frees b_pages if it was allocated.
 */
STATIC void
_xfs_buf_free_pages(
    xfs_buf_t   *bp)
{
    if (bp->b_pages != bp->b_page_array) {
        kmem_free(bp->b_pages);
        bp->b_pages = NULL;
    }
}

/*
 *  Releases the specified buffer.
 *
 *  The modification state of any associated pages is left unchanged.
 *  The buffer most not be on any hash - use xfs_buf_rele instead for
 *  hashed and refcounted buffers
 */
void
xfs_buf_free(
    xfs_buf_t       *bp)
{
    trace_xfs_buf_free(bp, _RET_IP_);

    ASSERT(list_empty(&bp->b_lru));

    if (bp->b_flags & _XBF_PAGES) {
        uint        i;

        if (xfs_buf_is_vmapped(bp))
            vm_unmap_ram(bp->b_addr - bp->b_offset,
                    bp->b_page_count);

        for (i = 0; i < bp->b_page_count; i++) {
            struct page *page = bp->b_pages[i];

            __free_page(page);
        }
    } else if (bp->b_flags & _XBF_KMEM)
        kmem_free(bp->b_addr);
    _xfs_buf_free_pages(bp);
    xfs_buf_free_maps(bp);
    kmem_zone_free(xfs_buf_zone, bp);
}

/*
 * Allocates all the pages for buffer in question and builds it's page list.
 */
STATIC int
xfs_buf_allocate_memory(
    xfs_buf_t       *bp,
    uint            flags)
{
    size_t          size;
    size_t          nbytes, offset;
    gfp_t           gfp_mask = xb_to_gfp(flags);
    unsigned short      page_count, i;
    xfs_off_t       start, end;
    int         error;

    /*
     * for buffers that are contained within a single page, just allocate
     * the memory from the heap - there's no need for the complexity of
     * page arrays to keep allocation down to order 0.
     */
    size = BBTOB(bp->b_length);
    if (size < PAGE_SIZE) {
        bp->b_addr = kmem_alloc(size, KM_NOFS);
        if (!bp->b_addr) {
            /* low memory - use alloc_page loop instead */
            goto use_alloc_page;
        }

        if (((unsigned long)(bp->b_addr + size - 1) & PAGE_MASK) !=
            ((unsigned long)bp->b_addr & PAGE_MASK)) {
            /* b_addr spans two pages - use alloc_page instead */
            kmem_free(bp->b_addr);
            bp->b_addr = NULL;
            goto use_alloc_page;
        }
        bp->b_offset = offset_in_page(bp->b_addr);
        bp->b_pages = bp->b_page_array;
        bp->b_pages[0] = virt_to_page(bp->b_addr);
        bp->b_page_count = 1;
        bp->b_flags |= _XBF_KMEM;
        return 0;
    }

use_alloc_page:
    start = BBTOB(bp->b_map.bm_bn) >> PAGE_SHIFT;
    end = (BBTOB(bp->b_map.bm_bn + bp->b_length) + PAGE_SIZE - 1)
                                >> PAGE_SHIFT;
    page_count = end - start;
    error = _xfs_buf_get_pages(bp, page_count, flags);
    if (unlikely(error))
        return error;

    offset = bp->b_offset;
    bp->b_flags |= _XBF_PAGES;

    for (i = 0; i < bp->b_page_count; i++) {
        struct page *page;
        uint        retries = 0;
retry:
        page = alloc_page(gfp_mask);
        if (unlikely(page == NULL)) {
            if (flags & XBF_READ_AHEAD) {
                bp->b_page_count = i;
                error = ENOMEM;
                goto out_free_pages;
            }

            /*
             * This could deadlock.
             *
             * But until all the XFS lowlevel code is revamped to
             * handle buffer allocation failures we can't do much.
             */
            if (!(++retries % 100))
                xfs_err(NULL,
        "possible memory allocation deadlock in %s (mode:0x%x)",
                    __func__, gfp_mask);

            XFS_STATS_INC(xb_page_retries);
            congestion_wait(BLK_RW_ASYNC, HZ/50);
            goto retry;
        }

        XFS_STATS_INC(xb_page_found);

        nbytes = min_t(size_t, size, PAGE_SIZE - offset);
        size -= nbytes;
        bp->b_pages[i] = page;
        offset = 0;
    }
    return 0;

out_free_pages:
    for (i = 0; i < bp->b_page_count; i++)
        __free_page(bp->b_pages[i]);
    return error;
}

/*
 *  Map buffer into kernel address-space if necessary.
 */
STATIC int
_xfs_buf_map_pages(
    xfs_buf_t       *bp,
    uint            flags)
{
    ASSERT(bp->b_flags & _XBF_PAGES);
    if (bp->b_page_count == 1) {
        /* A single page buffer is always mappable */
        bp->b_addr = page_address(bp->b_pages[0]) + bp->b_offset;
    } else if (flags & XBF_UNMAPPED) {
        bp->b_addr = NULL;
    } else {
        int retried = 0;

        do {
            bp->b_addr = vm_map_ram(bp->b_pages, bp->b_page_count,
                        -1, PAGE_KERNEL);
            if (bp->b_addr)
                break;
            vm_unmap_aliases();
        } while (retried++ <= 1);

        if (!bp->b_addr)
            return -ENOMEM;
        bp->b_addr += bp->b_offset;
    }

    return 0;
}

/*
 *  Finding and Reading Buffers
 */

/*
 *  Look up, and creates if absent, a lockable buffer for
 *  a given range of an inode.  The buffer is returned
 *  locked. No I/O is implied by this call.
 */
xfs_buf_t *
_xfs_buf_find(
    struct xfs_buftarg  *btp,
    struct xfs_buf_map  *map,
    int         nmaps,
    xfs_buf_flags_t     flags,
    xfs_buf_t       *new_bp)
{
    size_t          numbytes;
    struct xfs_perag    *pag;
    struct rb_node      **rbp;
    struct rb_node      *parent;
    xfs_buf_t       *bp;
    xfs_daddr_t     blkno = map[0].bm_bn;
    int         numblks = 0;
    int         i;

    for (i = 0; i < nmaps; i++)
        numblks += map[i].bm_len;
    numbytes = BBTOB(numblks);

    /* Check for IOs smaller than the sector size / not sector aligned */
    ASSERT(!(numbytes < (1 << btp->bt_sshift)));
    ASSERT(!(BBTOB(blkno) & (xfs_off_t)btp->bt_smask));

    /* get tree root */
    pag = xfs_perag_get(btp->bt_mount,
                xfs_daddr_to_agno(btp->bt_mount, blkno));

    /* walk tree */
    spin_lock(&pag->pag_buf_lock);
    rbp = &pag->pag_buf_tree.rb_node;
    parent = NULL;
    bp = NULL;
    while (*rbp) {
        parent = *rbp;
        bp = rb_entry(parent, struct xfs_buf, b_rbnode);

        if (blkno < bp->b_bn)
            rbp = &(*rbp)->rb_left;
        else if (blkno > bp->b_bn)
            rbp = &(*rbp)->rb_right;
        else {
            /*
             * found a block number match. If the range doesn't
             * match, the only way this is allowed is if the buffer
             * in the cache is stale and the transaction that made
             * it stale has not yet committed. i.e. we are
             * reallocating a busy extent. Skip this buffer and
             * continue searching to the right for an exact match.
             */
            if (bp->b_length != numblks) {
                ASSERT(bp->b_flags & XBF_STALE);
                rbp = &(*rbp)->rb_right;
                continue;
            }
            atomic_inc(&bp->b_hold);
            goto found;
        }
    }

    /* No match found */
    if (new_bp) {
        rb_link_node(&new_bp->b_rbnode, parent, rbp);
        rb_insert_color(&new_bp->b_rbnode, &pag->pag_buf_tree);
        /* the buffer keeps the perag reference until it is freed */
        new_bp->b_pag = pag;
        spin_unlock(&pag->pag_buf_lock);
    } else {
        XFS_STATS_INC(xb_miss_locked);
        spin_unlock(&pag->pag_buf_lock);
        xfs_perag_put(pag);
    }
    return new_bp;

found:
    spin_unlock(&pag->pag_buf_lock);
    xfs_perag_put(pag);

    if (!xfs_buf_trylock(bp)) {
        if (flags & XBF_TRYLOCK) {
            xfs_buf_rele(bp);
            XFS_STATS_INC(xb_busy_locked);
            return NULL;
        }
        xfs_buf_lock(bp);
        XFS_STATS_INC(xb_get_locked_waited);
    }

    /*
     * if the buffer is stale, clear all the external state associated with
     * it. We need to keep flags such as how we allocated the buffer memory
     * intact here.
     */
    if (bp->b_flags & XBF_STALE) {
        ASSERT((bp->b_flags & _XBF_DELWRI_Q) == 0);
        bp->b_flags &= _XBF_KMEM | _XBF_PAGES;
    }

    trace_xfs_buf_find(bp, flags, _RET_IP_);
    XFS_STATS_INC(xb_get_locked);
    return bp;
}

/*
 * Assembles a buffer covering the specified range. The code is optimised for
 * cache hits, as metadata intensive workloads will see 3 orders of magnitude
 * more hits than misses.
 */
struct xfs_buf *
xfs_buf_get_map(
    struct xfs_buftarg  *target,
    struct xfs_buf_map  *map,
    int         nmaps,
    xfs_buf_flags_t     flags)
{
    struct xfs_buf      *bp;
    struct xfs_buf      *new_bp;
    int         error = 0;

    bp = _xfs_buf_find(target, map, nmaps, flags, NULL);
    if (likely(bp))
        goto found;

    new_bp = _xfs_buf_alloc(target, map, nmaps, flags);
    if (unlikely(!new_bp))
        return NULL;

    error = xfs_buf_allocate_memory(new_bp, flags);
    if (error) {
        xfs_buf_free(new_bp);
        return NULL;
    }

    bp = _xfs_buf_find(target, map, nmaps, flags, new_bp);
    if (!bp) {
        xfs_buf_free(new_bp);
        return NULL;
    }

    if (bp != new_bp)
        xfs_buf_free(new_bp);

found:
    if (!bp->b_addr) {
        error = _xfs_buf_map_pages(bp, flags);
        if (unlikely(error)) {
            xfs_warn(target->bt_mount,
                "%s: failed to map pages\n", __func__);
            xfs_buf_relse(bp);
            return NULL;
        }
    }

    XFS_STATS_INC(xb_get);
    trace_xfs_buf_get(bp, flags, _RET_IP_);
    return bp;
}

STATIC int
_xfs_buf_read(
    xfs_buf_t       *bp,
    xfs_buf_flags_t     flags)
{
    ASSERT(!(flags & XBF_WRITE));
    ASSERT(bp->b_map.bm_bn != XFS_BUF_DADDR_NULL);

    bp->b_flags &= ~(XBF_WRITE | XBF_ASYNC | XBF_READ_AHEAD);
    bp->b_flags |= flags & (XBF_READ | XBF_ASYNC | XBF_READ_AHEAD);

    xfs_buf_iorequest(bp);
    if (flags & XBF_ASYNC)
        return 0;
    return xfs_buf_iowait(bp);
}

xfs_buf_t *
xfs_buf_read_map(
    struct xfs_buftarg  *target,
    struct xfs_buf_map  *map,
    int         nmaps,
    xfs_buf_flags_t     flags)
{
    struct xfs_buf      *bp;

    flags |= XBF_READ;

    bp = xfs_buf_get_map(target, map, nmaps, flags);
    if (bp) {
        trace_xfs_buf_read(bp, flags, _RET_IP_);

        if (!XFS_BUF_ISDONE(bp)) {
            XFS_STATS_INC(xb_get_read);
            _xfs_buf_read(bp, flags);
        } else if (flags & XBF_ASYNC) {
            /*
             * Read ahead call which is already satisfied,
             * drop the buffer
             */
            xfs_buf_relse(bp);
            return NULL;
        } else {
            /* We do not want read in the flags */
            bp->b_flags &= ~XBF_READ;
        }
    }

    return bp;
}

/*
 *  If we are not low on memory then do the readahead in a deadlock
 *  safe manner.
 */
void
xfs_buf_readahead_map(
    struct xfs_buftarg  *target,
    struct xfs_buf_map  *map,
    int         nmaps)
{
    if (bdi_read_congested(target->bt_bdi))
        return;

    xfs_buf_read_map(target, map, nmaps,
             XBF_TRYLOCK|XBF_ASYNC|XBF_READ_AHEAD);
}

/*
 * Read an uncached buffer from disk. Allocates and returns a locked
 * buffer containing the disk contents or nothing.
 */
struct xfs_buf *
xfs_buf_read_uncached(
    struct xfs_buftarg  *target,
    xfs_daddr_t     daddr,
    size_t          numblks,
    int         flags)
{
    xfs_buf_t       *bp;
    int         error;

    bp = xfs_buf_get_uncached(target, numblks, flags);
    if (!bp)
        return NULL;

    /* set up the buffer for a read IO */
    ASSERT(bp->b_map_count == 1);
    bp->b_bn = daddr;
    bp->b_maps[0].bm_bn = daddr;
    bp->b_flags |= XBF_READ;

    xfsbdstrat(target->bt_mount, bp);
    error = xfs_buf_iowait(bp);
    if (error) {
        xfs_buf_relse(bp);
        return NULL;
    }
    return bp;
}

/*
 * Return a buffer allocated as an empty buffer and associated to external
 * memory via xfs_buf_associate_memory() back to it's empty state.
 */
void
xfs_buf_set_empty(
    struct xfs_buf      *bp,
    size_t          numblks)
{
    if (bp->b_pages)
        _xfs_buf_free_pages(bp);

    bp->b_pages = NULL;
    bp->b_page_count = 0;
    bp->b_addr = NULL;
    bp->b_length = numblks;
    bp->b_io_length = numblks;

    ASSERT(bp->b_map_count == 1);
    bp->b_bn = XFS_BUF_DADDR_NULL;
    bp->b_maps[0].bm_bn = XFS_BUF_DADDR_NULL;
    bp->b_maps[0].bm_len = bp->b_length;
}

static inline struct page *
mem_to_page(
    void            *addr)
{
    if ((!is_vmalloc_addr(addr))) {
        return virt_to_page(addr);
    } else {
        return vmalloc_to_page(addr);
    }
}

int
xfs_buf_associate_memory(
    xfs_buf_t       *bp,
    void            *mem,
    size_t          len)
{
    int         rval;
    int         i = 0;
    unsigned long       pageaddr;
    unsigned long       offset;
    size_t          buflen;
    int         page_count;

    pageaddr = (unsigned long)mem & PAGE_MASK;
    offset = (unsigned long)mem - pageaddr;
    buflen = PAGE_ALIGN(len + offset);
    page_count = buflen >> PAGE_SHIFT;

    /* Free any previous set of page pointers */
    if (bp->b_pages)
        _xfs_buf_free_pages(bp);

    bp->b_pages = NULL;
    bp->b_addr = mem;

    rval = _xfs_buf_get_pages(bp, page_count, 0);
    if (rval)
        return rval;

    bp->b_offset = offset;

    for (i = 0; i < bp->b_page_count; i++) {
        bp->b_pages[i] = mem_to_page((void *)pageaddr);
        pageaddr += PAGE_SIZE;
    }

    bp->b_io_length = BTOBB(len);
    bp->b_length = BTOBB(buflen);

    return 0;
}

xfs_buf_t *
xfs_buf_get_uncached(
    struct xfs_buftarg  *target,
    size_t          numblks,
    int         flags)
{
    unsigned long       page_count;
    int         error, i;
    struct xfs_buf      *bp;
    DEFINE_SINGLE_BUF_MAP(map, XFS_BUF_DADDR_NULL, numblks);

    bp = _xfs_buf_alloc(target, &map, 1, 0);
    if (unlikely(bp == NULL))
        goto fail;

    page_count = PAGE_ALIGN(numblks << BBSHIFT) >> PAGE_SHIFT;
    error = _xfs_buf_get_pages(bp, page_count, 0);
    if (error)
        goto fail_free_buf;

    for (i = 0; i < page_count; i++) {
        bp->b_pages[i] = alloc_page(xb_to_gfp(flags));
        if (!bp->b_pages[i])
            goto fail_free_mem;
    }
    bp->b_flags |= _XBF_PAGES;

    error = _xfs_buf_map_pages(bp, 0);
    if (unlikely(error)) {
        xfs_warn(target->bt_mount,
            "%s: failed to map pages\n", __func__);
        goto fail_free_mem;
    }

    trace_xfs_buf_get_uncached(bp, _RET_IP_);
    return bp;

 fail_free_mem:
    while (--i >= 0)
        __free_page(bp->b_pages[i]);
    _xfs_buf_free_pages(bp);
 fail_free_buf:
    xfs_buf_free_maps(bp);
    kmem_zone_free(xfs_buf_zone, bp);
 fail:
    return NULL;
}

/*
 *  Increment reference count on buffer, to hold the buffer concurrently
 *  with another thread which may release (free) the buffer asynchronously.
 *  Must hold the buffer already to call this function.
 */
void
xfs_buf_hold(
    xfs_buf_t       *bp)
{
    trace_xfs_buf_hold(bp, _RET_IP_);
    atomic_inc(&bp->b_hold);
}

/*
 *  Releases a hold on the specified buffer.  If the
 *  the hold count is 1, calls xfs_buf_free.
 */
void
xfs_buf_rele(
    xfs_buf_t       *bp)
{
    struct xfs_perag    *pag = bp->b_pag;

    trace_xfs_buf_rele(bp, _RET_IP_);

    if (!pag) {
        ASSERT(list_empty(&bp->b_lru));
        ASSERT(RB_EMPTY_NODE(&bp->b_rbnode));
        if (atomic_dec_and_test(&bp->b_hold))
            xfs_buf_free(bp);
        return;
    }

    ASSERT(!RB_EMPTY_NODE(&bp->b_rbnode));

    ASSERT(atomic_read(&bp->b_hold) > 0);
    if (atomic_dec_and_lock(&bp->b_hold, &pag->pag_buf_lock)) {
        if (!(bp->b_flags & XBF_STALE) &&
               atomic_read(&bp->b_lru_ref)) {
            xfs_buf_lru_add(bp);
            spin_unlock(&pag->pag_buf_lock);
        } else {
            xfs_buf_lru_del(bp);
            ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));
            rb_erase(&bp->b_rbnode, &pag->pag_buf_tree);
            spin_unlock(&pag->pag_buf_lock);
            xfs_perag_put(pag);
            xfs_buf_free(bp);
        }
    }
}


/*
 *  Lock a buffer object, if it is not already locked.
 *
 *  If we come across a stale, pinned, locked buffer, we know that we are
 *  being asked to lock a buffer that has been reallocated. Because it is
 *  pinned, we know that the log has not been pushed to disk and hence it
 *  will still be locked.  Rather than continuing to have trylock attempts
 *  fail until someone else pushes the log, push it ourselves before
 *  returning.  This means that the xfsaild will not get stuck trying
 *  to push on stale inode buffers.
 */
int
xfs_buf_trylock(
    struct xfs_buf      *bp)
{
    int         locked;

    locked = down_trylock(&bp->b_sema) == 0;
    if (locked)
        XB_SET_OWNER(bp);
    else if (atomic_read(&bp->b_pin_count) && (bp->b_flags & XBF_STALE))
        xfs_log_force(bp->b_target->bt_mount, 0);

    trace_xfs_buf_trylock(bp, _RET_IP_);
    return locked;
}

/*
 *  Lock a buffer object.
 *
 *  If we come across a stale, pinned, locked buffer, we know that we
 *  are being asked to lock a buffer that has been reallocated. Because
 *  it is pinned, we know that the log has not been pushed to disk and
 *  hence it will still be locked. Rather than sleeping until someone
 *  else pushes the log, push it ourselves before trying to get the lock.
 */
void
xfs_buf_lock(
    struct xfs_buf      *bp)
{
    trace_xfs_buf_lock(bp, _RET_IP_);

    if (atomic_read(&bp->b_pin_count) && (bp->b_flags & XBF_STALE))
        xfs_log_force(bp->b_target->bt_mount, 0);
    down(&bp->b_sema);
    XB_SET_OWNER(bp);

    trace_xfs_buf_lock_done(bp, _RET_IP_);
}

void
xfs_buf_unlock(
    struct xfs_buf      *bp)
{
    XB_CLEAR_OWNER(bp);
    up(&bp->b_sema);

    trace_xfs_buf_unlock(bp, _RET_IP_);
}

STATIC void
xfs_buf_wait_unpin(
    xfs_buf_t       *bp)
{
    DECLARE_WAITQUEUE   (wait, current);

    if (atomic_read(&bp->b_pin_count) == 0)
        return;

    add_wait_queue(&bp->b_waiters, &wait);
    for (;;) {
        set_current_state(TASK_UNINTERRUPTIBLE);
        if (atomic_read(&bp->b_pin_count) == 0)
            break;
        io_schedule();
    }
    remove_wait_queue(&bp->b_waiters, &wait);
    set_current_state(TASK_RUNNING);
}

/*
 *  Buffer Utility Routines
 */

STATIC void
xfs_buf_iodone_work(
    struct work_struct  *work)
{
    xfs_buf_t       *bp =
        container_of(work, xfs_buf_t, b_iodone_work);

    if (bp->b_iodone)
        (*(bp->b_iodone))(bp);
    else if (bp->b_flags & XBF_ASYNC)
        xfs_buf_relse(bp);
}

void
xfs_buf_ioend(
    xfs_buf_t       *bp,
    int         schedule)
{
    trace_xfs_buf_iodone(bp, _RET_IP_);

    bp->b_flags &= ~(XBF_READ | XBF_WRITE | XBF_READ_AHEAD);
    if (bp->b_error == 0)
        bp->b_flags |= XBF_DONE;

    if ((bp->b_iodone) || (bp->b_flags & XBF_ASYNC)) {
        if (schedule) {
            INIT_WORK(&bp->b_iodone_work, xfs_buf_iodone_work);
            queue_work(xfslogd_workqueue, &bp->b_iodone_work);
        } else {
            xfs_buf_iodone_work(&bp->b_iodone_work);
        }
    } else {
        complete(&bp->b_iowait);
    }
}

void
xfs_buf_ioerror(
    xfs_buf_t       *bp,
    int         error)
{
    ASSERT(error >= 0 && error <= 0xffff);
    bp->b_error = (unsigned short)error;
    trace_xfs_buf_ioerror(bp, error, _RET_IP_);
}

void
xfs_buf_ioerror_alert(
    struct xfs_buf      *bp,
    const char      *func)
{
    xfs_alert(bp->b_target->bt_mount,
"metadata I/O error: block 0x%llx (\"%s\") error %d numblks %d",
        (__uint64_t)XFS_BUF_ADDR(bp), func, bp->b_error, bp->b_length);
}

/*
 * Called when we want to stop a buffer from getting written or read.
 * We attach the EIO error, muck with its flags, and call xfs_buf_ioend
 * so that the proper iodone callbacks get called.
 */
STATIC int
xfs_bioerror(
    xfs_buf_t *bp)
{
#ifdef XFSERRORDEBUG
    ASSERT(XFS_BUF_ISREAD(bp) || bp->b_iodone);
#endif

    /*
     * No need to wait until the buffer is unpinned, we aren't flushing it.
     */
    xfs_buf_ioerror(bp, EIO);

    /*
     * We're calling xfs_buf_ioend, so delete XBF_DONE flag.
     */
    XFS_BUF_UNREAD(bp);
    XFS_BUF_UNDONE(bp);
    xfs_buf_stale(bp);

    xfs_buf_ioend(bp, 0);

    return EIO;
}

/*
 * Same as xfs_bioerror, except that we are releasing the buffer
 * here ourselves, and avoiding the xfs_buf_ioend call.
 * This is meant for userdata errors; metadata bufs come with
 * iodone functions attached, so that we can track down errors.
 */
STATIC int
xfs_bioerror_relse(
    struct xfs_buf  *bp)
{
    int64_t     fl = bp->b_flags;
    /*
     * No need to wait until the buffer is unpinned.
     * We aren't flushing it.
     *
     * chunkhold expects B_DONE to be set, whether
     * we actually finish the I/O or not. We don't want to
     * change that interface.
     */
    XFS_BUF_UNREAD(bp);
    XFS_BUF_DONE(bp);
    xfs_buf_stale(bp);
    bp->b_iodone = NULL;
    if (!(fl & XBF_ASYNC)) {
        /*
         * Mark b_error and B_ERROR _both_.
         * Lot's of chunkcache code assumes that.
         * There's no reason to mark error for
         * ASYNC buffers.
         */
        xfs_buf_ioerror(bp, EIO);
        complete(&bp->b_iowait);
    } else {
        xfs_buf_relse(bp);
    }

    return EIO;
}

STATIC int
xfs_bdstrat_cb(
    struct xfs_buf  *bp)
{
    if (XFS_FORCED_SHUTDOWN(bp->b_target->bt_mount)) {
        trace_xfs_bdstrat_shut(bp, _RET_IP_);
        /*
         * Metadata write that didn't get logged but
         * written delayed anyway. These aren't associated
         * with a transaction, and can be ignored.
         */
        if (!bp->b_iodone && !XFS_BUF_ISREAD(bp))
            return xfs_bioerror_relse(bp);
        else
            return xfs_bioerror(bp);
    }

    xfs_buf_iorequest(bp);
    return 0;
}

int
xfs_bwrite(
    struct xfs_buf      *bp)
{
    int         error;

    ASSERT(xfs_buf_islocked(bp));

    bp->b_flags |= XBF_WRITE;
    bp->b_flags &= ~(XBF_ASYNC | XBF_READ | _XBF_DELWRI_Q);

    xfs_bdstrat_cb(bp);

    error = xfs_buf_iowait(bp);
    if (error) {
        xfs_force_shutdown(bp->b_target->bt_mount,
                   SHUTDOWN_META_IO_ERROR);
    }
    return error;
}

/*
 * Wrapper around bdstrat so that we can stop data from going to disk in case
 * we are shutting down the filesystem.  Typically user data goes thru this
 * path; one of the exceptions is the superblock.
 */
void
xfsbdstrat(
    struct xfs_mount    *mp,
    struct xfs_buf      *bp)
{
    if (XFS_FORCED_SHUTDOWN(mp)) {
        trace_xfs_bdstrat_shut(bp, _RET_IP_);
        xfs_bioerror_relse(bp);
        return;
    }

    xfs_buf_iorequest(bp);
}

STATIC void
_xfs_buf_ioend(
    xfs_buf_t       *bp,
    int         schedule)
{
    if (atomic_dec_and_test(&bp->b_io_remaining) == 1)
        xfs_buf_ioend(bp, schedule);
}

STATIC void
xfs_buf_bio_end_io(
    struct bio      *bio,
    int         error)
{
    xfs_buf_t       *bp = (xfs_buf_t *)bio->bi_private;

    /*
     * don't overwrite existing errors - otherwise we can lose errors on
     * buffers that require multiple bios to complete.
     */
    if (!bp->b_error)
        xfs_buf_ioerror(bp, -error);

    if (!bp->b_error && xfs_buf_is_vmapped(bp) && (bp->b_flags & XBF_READ))
        invalidate_kernel_vmap_range(bp->b_addr, xfs_buf_vmap_len(bp));

    _xfs_buf_ioend(bp, 1);
    bio_put(bio);
}

static void
xfs_buf_ioapply_map(
    struct xfs_buf  *bp,
    int     map,
    int     *buf_offset,
    int     *count,
    int     rw)
{
    int     page_index;
    int     total_nr_pages = bp->b_page_count;
    int     nr_pages;
    struct bio  *bio;
    sector_t    sector =  bp->b_maps[map].bm_bn;
    int     size;
    int     offset;

    total_nr_pages = bp->b_page_count;

    /* skip the pages in the buffer before the start offset */
    page_index = 0;
    offset = *buf_offset;
    while (offset >= PAGE_SIZE) {
        page_index++;
        offset -= PAGE_SIZE;
    }

    /*
     * Limit the IO size to the length of the current vector, and update the
     * remaining IO count for the next time around.
     */
    size = min_t(int, BBTOB(bp->b_maps[map].bm_len), *count);
    *count -= size;
    *buf_offset += size;

next_chunk:
    atomic_inc(&bp->b_io_remaining);
    nr_pages = BIO_MAX_SECTORS >> (PAGE_SHIFT - BBSHIFT);
    if (nr_pages > total_nr_pages)
        nr_pages = total_nr_pages;

    bio = bio_alloc(GFP_NOIO, nr_pages);
    bio->bi_bdev = bp->b_target->bt_bdev;
    bio->bi_sector = sector;
    bio->bi_end_io = xfs_buf_bio_end_io;
    bio->bi_private = bp;


    for (; size && nr_pages; nr_pages--, page_index++) {
        int rbytes, nbytes = PAGE_SIZE - offset;

        if (nbytes > size)
            nbytes = size;

        rbytes = bio_add_page(bio, bp->b_pages[page_index], nbytes,
                      offset);
        if (rbytes < nbytes)
            break;

        offset = 0;
        sector += BTOBB(nbytes);
        size -= nbytes;
        total_nr_pages--;
    }

    if (likely(bio->bi_size)) {
        if (xfs_buf_is_vmapped(bp)) {
            flush_kernel_vmap_range(bp->b_addr,
                        xfs_buf_vmap_len(bp));
        }
        submit_bio(rw, bio);
        if (size)
            goto next_chunk;
    } else {
        /*
         * This is guaranteed not to be the last io reference count
         * because the caller (xfs_buf_iorequest) holds a count itself.
         */
        atomic_dec(&bp->b_io_remaining);
        xfs_buf_ioerror(bp, EIO);
        bio_put(bio);
    }

}

STATIC void
_xfs_buf_ioapply(
    struct xfs_buf  *bp)
{
    struct blk_plug plug;
    int     rw;
    int     offset;
    int     size;
    int     i;

    if (bp->b_flags & XBF_WRITE) {
        if (bp->b_flags & XBF_SYNCIO)
            rw = WRITE_SYNC;
        else
            rw = WRITE;
        if (bp->b_flags & XBF_FUA)
            rw |= REQ_FUA;
        if (bp->b_flags & XBF_FLUSH)
            rw |= REQ_FLUSH;
    } else if (bp->b_flags & XBF_READ_AHEAD) {
        rw = READA;
    } else {
        rw = READ;
    }

    /* we only use the buffer cache for meta-data */
    rw |= REQ_META;

    /*
     * Walk all the vectors issuing IO on them. Set up the initial offset
     * into the buffer and the desired IO size before we start -
     * _xfs_buf_ioapply_vec() will modify them appropriately for each
     * subsequent call.
     */
    offset = bp->b_offset;
    size = BBTOB(bp->b_io_length);
    blk_start_plug(&plug);
    for (i = 0; i < bp->b_map_count; i++) {
        xfs_buf_ioapply_map(bp, i, &offset, &size, rw);
        if (bp->b_error)
            break;
        if (size <= 0)
            break;  /* all done */
    }
    blk_finish_plug(&plug);
}

void
xfs_buf_iorequest(
    xfs_buf_t       *bp)
{
    trace_xfs_buf_iorequest(bp, _RET_IP_);

    ASSERT(!(bp->b_flags & _XBF_DELWRI_Q));

    if (bp->b_flags & XBF_WRITE)
        xfs_buf_wait_unpin(bp);
    xfs_buf_hold(bp);

    /* Set the count to 1 initially, this will stop an I/O
     * completion callout which happens before we have started
     * all the I/O from calling xfs_buf_ioend too early.
     */
    atomic_set(&bp->b_io_remaining, 1);
    _xfs_buf_ioapply(bp);
    _xfs_buf_ioend(bp, 1);

    xfs_buf_rele(bp);
}

/*
 * Waits for I/O to complete on the buffer supplied.  It returns immediately if
 * no I/O is pending or there is already a pending error on the buffer.  It
 * returns the I/O error code, if any, or 0 if there was no error.
 */
int
xfs_buf_iowait(
    xfs_buf_t       *bp)
{
    trace_xfs_buf_iowait(bp, _RET_IP_);

    if (!bp->b_error)
        wait_for_completion(&bp->b_iowait);

    trace_xfs_buf_iowait_done(bp, _RET_IP_);
    return bp->b_error;
}

xfs_caddr_t
xfs_buf_offset(
    xfs_buf_t       *bp,
    size_t          offset)
{
    struct page     *page;

    if (bp->b_addr)
        return bp->b_addr + offset;

    offset += bp->b_offset;
    page = bp->b_pages[offset >> PAGE_SHIFT];
    return (xfs_caddr_t)page_address(page) + (offset & (PAGE_SIZE-1));
}

/*
 *  Move data into or out of a buffer.
 */
void
xfs_buf_iomove(
    xfs_buf_t       *bp,    /* buffer to process        */
    size_t          boff,   /* starting buffer offset   */
    size_t          bsize,  /* length to copy       */
    void            *data,  /* data address         */
    xfs_buf_rw_t        mode)   /* read/write/zero flag     */
{
    size_t          bend;

    bend = boff + bsize;
    while (boff < bend) {
        struct page *page;
        int     page_index, page_offset, csize;

        page_index = (boff + bp->b_offset) >> PAGE_SHIFT;
        page_offset = (boff + bp->b_offset) & ~PAGE_MASK;
        page = bp->b_pages[page_index];
        csize = min_t(size_t, PAGE_SIZE - page_offset,
                      BBTOB(bp->b_io_length) - boff);

        ASSERT((csize + page_offset) <= PAGE_SIZE);

        switch (mode) {
        case XBRW_ZERO:
            memset(page_address(page) + page_offset, 0, csize);
            break;
        case XBRW_READ:
            memcpy(data, page_address(page) + page_offset, csize);
            break;
        case XBRW_WRITE:
            memcpy(page_address(page) + page_offset, data, csize);
        }

        boff += csize;
        data += csize;
    }
}

/*
 *  Handling of buffer targets (buftargs).
 */

/*
 * Wait for any bufs with callbacks that have been submitted but have not yet
 * returned. These buffers will have an elevated hold count, so wait on those
 * while freeing all the buffers only held by the LRU.
 */
void
xfs_wait_buftarg(
    struct xfs_buftarg  *btp)
{
    struct xfs_buf      *bp;

restart:
    spin_lock(&btp->bt_lru_lock);
    while (!list_empty(&btp->bt_lru)) {
        bp = list_first_entry(&btp->bt_lru, struct xfs_buf, b_lru);
        if (atomic_read(&bp->b_hold) > 1) {
            spin_unlock(&btp->bt_lru_lock);
            delay(100);
            goto restart;
        }
        /*
         * clear the LRU reference count so the buffer doesn't get
         * ignored in xfs_buf_rele().
         */
        atomic_set(&bp->b_lru_ref, 0);
        spin_unlock(&btp->bt_lru_lock);
        xfs_buf_rele(bp);
        spin_lock(&btp->bt_lru_lock);
    }
    spin_unlock(&btp->bt_lru_lock);
}

int
xfs_buftarg_shrink(
    struct shrinker     *shrink,
    struct shrink_control   *sc)
{
    struct xfs_buftarg  *btp = container_of(shrink,
                    struct xfs_buftarg, bt_shrinker);
    struct xfs_buf      *bp;
    int nr_to_scan = sc->nr_to_scan;
    LIST_HEAD(dispose);

    if (!nr_to_scan)
        return btp->bt_lru_nr;

    spin_lock(&btp->bt_lru_lock);
    while (!list_empty(&btp->bt_lru)) {
        if (nr_to_scan-- <= 0)
            break;

        bp = list_first_entry(&btp->bt_lru, struct xfs_buf, b_lru);

        /*
         * Decrement the b_lru_ref count unless the value is already
         * zero. If the value is already zero, we need to reclaim the
         * buffer, otherwise it gets another trip through the LRU.
         */
        if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
            list_move_tail(&bp->b_lru, &btp->bt_lru);
            continue;
        }

        /*
         * remove the buffer from the LRU now to avoid needing another
         * lock round trip inside xfs_buf_rele().
         */
        list_move(&bp->b_lru, &dispose);
        btp->bt_lru_nr--;
        bp->b_lru_flags |= _XBF_LRU_DISPOSE;
    }
    spin_unlock(&btp->bt_lru_lock);

    while (!list_empty(&dispose)) {
        bp = list_first_entry(&dispose, struct xfs_buf, b_lru);
        list_del_init(&bp->b_lru);
        xfs_buf_rele(bp);
    }

    return btp->bt_lru_nr;
}

void
xfs_free_buftarg(
    struct xfs_mount    *mp,
    struct xfs_buftarg  *btp)
{
    unregister_shrinker(&btp->bt_shrinker);

    if (mp->m_flags & XFS_MOUNT_BARRIER)
        xfs_blkdev_issue_flush(btp);

    kmem_free(btp);
}

STATIC int
xfs_setsize_buftarg_flags(
    xfs_buftarg_t       *btp,
    unsigned int        blocksize,
    unsigned int        sectorsize,
    int         verbose)
{
    btp->bt_bsize = blocksize;
    btp->bt_sshift = ffs(sectorsize) - 1;
    btp->bt_smask = sectorsize - 1;

    if (set_blocksize(btp->bt_bdev, sectorsize)) {
        char name[BDEVNAME_SIZE];

        bdevname(btp->bt_bdev, name);

        xfs_warn(btp->bt_mount,
            "Cannot set_blocksize to %u on device %s\n",
            sectorsize, name);
        return EINVAL;
    }

    return 0;
}

/*
 *  When allocating the initial buffer target we have not yet
 *  read in the superblock, so don't know what sized sectors
 *  are being used is at this early stage.  Play safe.
 */
STATIC int
xfs_setsize_buftarg_early(
    xfs_buftarg_t       *btp,
    struct block_device *bdev)
{
    return xfs_setsize_buftarg_flags(btp,
            PAGE_SIZE, bdev_logical_block_size(bdev), 0);
}

int
xfs_setsize_buftarg(
    xfs_buftarg_t       *btp,
    unsigned int        blocksize,
    unsigned int        sectorsize)
{
    return xfs_setsize_buftarg_flags(btp, blocksize, sectorsize, 1);
}

xfs_buftarg_t *
xfs_alloc_buftarg(
    struct xfs_mount    *mp,
    struct block_device *bdev,
    int         external,
    const char      *fsname)
{
    xfs_buftarg_t       *btp;

    btp = kmem_zalloc(sizeof(*btp), KM_SLEEP);

    btp->bt_mount = mp;
    btp->bt_dev =  bdev->bd_dev;
    btp->bt_bdev = bdev;
    btp->bt_bdi = blk_get_backing_dev_info(bdev);
    if (!btp->bt_bdi)
        goto error;

    INIT_LIST_HEAD(&btp->bt_lru);
    spin_lock_init(&btp->bt_lru_lock);
    if (xfs_setsize_buftarg_early(btp, bdev))
        goto error;
    btp->bt_shrinker.shrink = xfs_buftarg_shrink;
    btp->bt_shrinker.seeks = DEFAULT_SEEKS;
    register_shrinker(&btp->bt_shrinker);
    return btp;

error:
    kmem_free(btp);
    return NULL;
}

/*
 * Add a buffer to the delayed write list.
 *
 * This queues a buffer for writeout if it hasn't already been.  Note that
 * neither this routine nor the buffer list submission functions perform
 * any internal synchronization.  It is expected that the lists are thread-local
 * to the callers.
 *
 * Returns true if we queued up the buffer, or false if it already had
 * been on the buffer list.
 */
bool
xfs_buf_delwri_queue(
    struct xfs_buf      *bp,
    struct list_head    *list)
{
    ASSERT(xfs_buf_islocked(bp));
    ASSERT(!(bp->b_flags & XBF_READ));

    /*
     * If the buffer is already marked delwri it already is queued up
     * by someone else for imediate writeout.  Just ignore it in that
     * case.
     */
    if (bp->b_flags & _XBF_DELWRI_Q) {
        trace_xfs_buf_delwri_queued(bp, _RET_IP_);
        return false;
    }

    trace_xfs_buf_delwri_queue(bp, _RET_IP_);

    /*
     * If a buffer gets written out synchronously or marked stale while it
     * is on a delwri list we lazily remove it. To do this, the other party
     * clears the  _XBF_DELWRI_Q flag but otherwise leaves the buffer alone.
     * It remains referenced and on the list.  In a rare corner case it
     * might get readded to a delwri list after the synchronous writeout, in
     * which case we need just need to re-add the flag here.
     */
    bp->b_flags |= _XBF_DELWRI_Q;
    if (list_empty(&bp->b_list)) {
        atomic_inc(&bp->b_hold);
        list_add_tail(&bp->b_list, list);
    }

    return true;
}

/*
 * Compare function is more complex than it needs to be because
 * the return value is only 32 bits and we are doing comparisons
 * on 64 bit values
 */
static int
xfs_buf_cmp(
    void        *priv,
    struct list_head *a,
    struct list_head *b)
{
    struct xfs_buf  *ap = container_of(a, struct xfs_buf, b_list);
    struct xfs_buf  *bp = container_of(b, struct xfs_buf, b_list);
    xfs_daddr_t     diff;

    diff = ap->b_map.bm_bn - bp->b_map.bm_bn;
    if (diff < 0)
        return -1;
    if (diff > 0)
        return 1;
    return 0;
}

static int
__xfs_buf_delwri_submit(
    struct list_head    *buffer_list,
    struct list_head    *io_list,
    bool            wait)
{
    struct blk_plug     plug;
    struct xfs_buf      *bp, *n;
    int         pinned = 0;

    list_for_each_entry_safe(bp, n, buffer_list, b_list) {
        if (!wait) {
            if (xfs_buf_ispinned(bp)) {
                pinned++;
                continue;
            }
            if (!xfs_buf_trylock(bp))
                continue;
        } else {
            xfs_buf_lock(bp);
        }

        /*
         * Someone else might have written the buffer synchronously or
         * marked it stale in the meantime.  In that case only the
         * _XBF_DELWRI_Q flag got cleared, and we have to drop the
         * reference and remove it from the list here.
         */
        if (!(bp->b_flags & _XBF_DELWRI_Q)) {
            list_del_init(&bp->b_list);
            xfs_buf_relse(bp);
            continue;
        }

        list_move_tail(&bp->b_list, io_list);
        trace_xfs_buf_delwri_split(bp, _RET_IP_);
    }

    list_sort(NULL, io_list, xfs_buf_cmp);

    blk_start_plug(&plug);
    list_for_each_entry_safe(bp, n, io_list, b_list) {
        bp->b_flags &= ~(_XBF_DELWRI_Q | XBF_ASYNC);
        bp->b_flags |= XBF_WRITE;

        if (!wait) {
            bp->b_flags |= XBF_ASYNC;
            list_del_init(&bp->b_list);
        }
        xfs_bdstrat_cb(bp);
    }
    blk_finish_plug(&plug);

    return pinned;
}

/*
 * Write out a buffer list asynchronously.
 *
 * This will take the @buffer_list, write all non-locked and non-pinned buffers
 * out and not wait for I/O completion on any of the buffers.  This interface
 * is only safely useable for callers that can track I/O completion by higher
 * level means, e.g. AIL pushing as the @buffer_list is consumed in this
 * function.
 */
int
xfs_buf_delwri_submit_nowait(
    struct list_head    *buffer_list)
{
    LIST_HEAD       (io_list);
    return __xfs_buf_delwri_submit(buffer_list, &io_list, false);
}

/*
 * Write out a buffer list synchronously.
 *
 * This will take the @buffer_list, write all buffers out and wait for I/O
 * completion on all of the buffers. @buffer_list is consumed by the function,
 * so callers must have some other way of tracking buffers if they require such
 * functionality.
 */
int
xfs_buf_delwri_submit(
    struct list_head    *buffer_list)
{
    LIST_HEAD       (io_list);
    int         error = 0, error2;
    struct xfs_buf      *bp;

    __xfs_buf_delwri_submit(buffer_list, &io_list, true);

    /* Wait for IO to complete. */
    while (!list_empty(&io_list)) {
        bp = list_first_entry(&io_list, struct xfs_buf, b_list);

        list_del_init(&bp->b_list);
        error2 = xfs_buf_iowait(bp);
        xfs_buf_relse(bp);
        if (!error)
            error = error2;
    }

    return error;
}

int __init
xfs_buf_init(void)
{
    xfs_buf_zone = kmem_zone_init_flags(sizeof(xfs_buf_t), "xfs_buf",
                        KM_ZONE_HWALIGN, NULL);
    if (!xfs_buf_zone)
        goto out;

    xfslogd_workqueue = alloc_workqueue("xfslogd",
                    WQ_MEM_RECLAIM | WQ_HIGHPRI, 1);
    if (!xfslogd_workqueue)
        goto out_free_buf_zone;

    return 0;

 out_free_buf_zone:
    kmem_zone_destroy(xfs_buf_zone);
 out:
    return -ENOMEM;
}

void
xfs_buf_terminate(void)
{
    destroy_workqueue(xfslogd_workqueue);
    kmem_zone_destroy(xfs_buf_zone);
}