linux/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++;
	}
	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)) {
			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);
}

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;
}

/*
 * 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;
}


/*
 * All xfs metadata buffers except log state machine buffers
 * get this attached as their b_bdstrat callback function.
 * This is so that we can catch a buffer
 * after prematurely unpinning it to forcibly shutdown the filesystem.
 */
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;
}

/*
 * 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, 0);

	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--;
	}
	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);
}