xfs: fix race while discarding buffers [V4]

[deliverable/linux.git] / fs / xfs / xfs_buf.c

/*
 * 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;

        xfs_buf_ioerror(bp, -error);

        if (!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 {
                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);
}