[PATCH 0/2] xfs; fixes for 2.7.37-rc4

[PATCH 0/2] xfs; fixes for 2.7.37-rc4

[Dave Chinner]

The following two patches are bugs fixes needed for 2.6.37. The first fixes the
write truncation problem, and has been updated to pass blocks rather than byte
ranges  into the punch function. The second fixes a log hang triggered by AIL
pushing being unable to move a locked, pinned and stale buffer from the tail of
the log and hence free up log space. Both of these fixes are candidates for a
2.6.36.y stable release.

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[PATCH 1/2] xfs: fix failed write truncation handling.

[Dave Chinner]

From: Dave Chinner <dchinner@redhat.com>

Since the move to the new truncate sequence we call xfs_setattr to
truncate down excessively instanciated blocks.  As shown by the testcase
in kernel.org BZ #22452 that doesn't work too well.  Due to the confusion
of the internal inode size, and the VFS inode i_size it zeroes data that
it shouldn't.

But full blown truncate seems like overkill here.  We only instanciate
delayed allocations in the write path, and given that we never released
the iolock we can't have converted them to real allocations yet either.

The only nasty case is pre-existing preallocation which we need to skip.
We already do this for page discard during writeback, so make the delayed
allocation block punching a generic function and call it from the failed
write path as well as xfs_aops_discard_page. The callers are
responsible for ensuring that partial blocks are not truncated away,
and that they hold the ilock.

Based on a fix originally from Christoph Hellwig. This version used
filesystem blocks as the range unit.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/linux-2.6/xfs_aops.c |   94 ++++++++++++++++++------------------------
 fs/xfs/xfs_bmap.c           |   76 ++++++++++++++++++++++++++++++++++
 fs/xfs/xfs_bmap.h           |    5 ++
 3 files changed, 121 insertions(+), 54 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_aops.c b/fs/xfs/linux-2.6/xfs_aops.c
index 7d287af..691f612 100644
--- a/fs/xfs/linux-2.6/xfs_aops.c
+++ b/fs/xfs/linux-2.6/xfs_aops.c
@@ -934,7 +934,6 @@ xfs_aops_discard_page(
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct buffer_head	*bh, *head;
 	loff_t			offset = page_offset(page);
-	ssize_t			len = 1 << inode->i_blkbits;
 
 	if (!xfs_is_delayed_page(page, IO_DELAY))
 		goto out_invalidate;
@@ -949,58 +948,14 @@ xfs_aops_discard_page(
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 	bh = head = page_buffers(page);
 	do {
-		int		done;
-		xfs_fileoff_t	offset_fsb;
-		xfs_bmbt_irec_t	imap;
-		int		nimaps = 1;
 		int		error;
-		xfs_fsblock_t	firstblock;
-		xfs_bmap_free_t flist;
+		xfs_fileoff_t	start_fsb;
 
 		if (!buffer_delay(bh))
 			goto next_buffer;
 
-		offset_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
-
-		/*
-		 * Map the range first and check that it is a delalloc extent
-		 * before trying to unmap the range. Otherwise we will be
-		 * trying to remove a real extent (which requires a
-		 * transaction) or a hole, which is probably a bad idea...
-		 */
-		error = xfs_bmapi(NULL, ip, offset_fsb, 1,
-				XFS_BMAPI_ENTIRE,  NULL, 0, &imap,
-				&nimaps, NULL);
-
-		if (error) {
-			/* something screwed, just bail */
-			if (!XFS_FORCED_SHUTDOWN(ip->i_mount)) {
-				xfs_fs_cmn_err(CE_ALERT, ip->i_mount,
-				"page discard failed delalloc mapping lookup.");
-			}
-			break;
-		}
-		if (!nimaps) {
-			/* nothing there */
-			goto next_buffer;
-		}
-		if (imap.br_startblock != DELAYSTARTBLOCK) {
-			/* been converted, ignore */
-			goto next_buffer;
-		}
-		WARN_ON(imap.br_blockcount == 0);
-
-		/*
-		 * Note: while we initialise the firstblock/flist pair, they
-		 * should never be used because blocks should never be
-		 * allocated or freed for a delalloc extent and hence we need
-		 * don't cancel or finish them after the xfs_bunmapi() call.
-		 */
-		xfs_bmap_init(&flist, &firstblock);
-		error = xfs_bunmapi(NULL, ip, offset_fsb, 1, 0, 1, &firstblock,
-					&flist, &done);
-
-		ASSERT(!flist.xbf_count && !flist.xbf_first);
+		start_fsb = XFS_B_TO_FSBT(ip->i_mount, offset);
+		error = xfs_bmap_punch_delalloc_range(ip, start_fsb, 1);
 		if (error) {
 			/* something screwed, just bail */
 			if (!XFS_FORCED_SHUTDOWN(ip->i_mount)) {
@@ -1010,7 +965,7 @@ xfs_aops_discard_page(
 			break;
 		}
 next_buffer:
-		offset += len;
+		offset += 1 << inode->i_blkbits;
 
 	} while ((bh = bh->b_this_page) != head);
 
@@ -1505,11 +1460,42 @@ xfs_vm_write_failed(
 	struct inode		*inode = mapping->host;
 
 	if (to > inode->i_size) {
-		struct iattr	ia = {
-			.ia_valid	= ATTR_SIZE | ATTR_FORCE,
-			.ia_size	= inode->i_size,
-		};
-		xfs_setattr(XFS_I(inode), &ia, XFS_ATTR_NOLOCK);
+		/*
+		 * punch out the delalloc blocks we have already allocated. We
+		 * don't call xfs_setattr() to do this as we may be in the
+		 * middle of a multi-iovec write and so the vfs inode->i_size
+		 * will not match the xfs ip->i_size and so it will zero too
+		 * much. Hence we jus truncate the page cache to zero what is
+		 * necessary and punch the delalloc blocks directly.
+		 */
+		struct xfs_inode	*ip = XFS_I(inode);
+		xfs_fileoff_t		start_fsb;
+		xfs_fileoff_t		end_fsb;
+		int			error;
+
+		truncate_pagecache(inode, to, inode->i_size);
+
+		/*
+		 * Check if there are any blocks that are outside of i_size
+		 * that need to be trimmed back.
+		 */
+		start_fsb = XFS_B_TO_FSB(ip->i_mount, inode->i_size) + 1;
+		end_fsb = XFS_B_TO_FSB(ip->i_mount, to);
+		if (end_fsb <= start_fsb)
+			return;
+
+		xfs_ilock(ip, XFS_ILOCK_EXCL);
+		error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
+							end_fsb - start_fsb);
+		if (error) {
+			/* something screwed, just bail */
+			if (!XFS_FORCED_SHUTDOWN(ip->i_mount)) {
+				xfs_fs_cmn_err(CE_ALERT, ip->i_mount,
+			"xfs_vm_write_failed: unable to clean up ino %lld",
+						ip->i_ino);
+			}
+		}
+		xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	}
 }
 
diff --git a/fs/xfs/xfs_bmap.c b/fs/xfs/xfs_bmap.c
index 8abd12e..08b179f 100644
--- a/fs/xfs/xfs_bmap.c
+++ b/fs/xfs/xfs_bmap.c
@@ -6070,3 +6070,79 @@ xfs_bmap_disk_count_leaves(
 		*count += xfs_bmbt_disk_get_blockcount(frp);
 	}
 }
+
+/*
+ * dead simple method of punching delalyed allocation blocks from a range in
+ * the inode. Walks a block at a time so will be slow, but is only executed in
+ * rare error cases so the overhead is not critical. This will alays punch out
+ * both the start and end blocks, even if the ranges only partially overlap
+ * them, so it is up to the caller to ensure that partial blocks are not
+ * passed in.
+ */
+int
+xfs_bmap_punch_delalloc_range(
+	struct xfs_inode	*ip,
+	xfs_fileoff_t		start_fsb,
+	xfs_fileoff_t		length)
+{
+	xfs_fileoff_t		remaining = length;
+	int			error = 0;
+
+	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
+
+	do {
+		int		done;
+		xfs_bmbt_irec_t	imap;
+		int		nimaps = 1;
+		xfs_fsblock_t	firstblock;
+		xfs_bmap_free_t flist;
+
+		/*
+		 * Map the range first and check that it is a delalloc extent
+		 * before trying to unmap the range. Otherwise we will be
+		 * trying to remove a real extent (which requires a
+		 * transaction) or a hole, which is probably a bad idea...
+		 */
+		error = xfs_bmapi(NULL, ip, start_fsb, 1,
+				XFS_BMAPI_ENTIRE,  NULL, 0, &imap,
+				&nimaps, NULL);
+
+		if (error) {
+			/* something screwed, just bail */
+			if (!XFS_FORCED_SHUTDOWN(ip->i_mount)) {
+				xfs_fs_cmn_err(CE_ALERT, ip->i_mount,
+			"Failed delalloc mapping lookup ino %lld fsb %lld.",
+						ip->i_ino, start_fsb);
+			}
+			break;
+		}
+		if (!nimaps) {
+			/* nothing there */
+			goto next_block;
+		}
+		if (imap.br_startblock != DELAYSTARTBLOCK) {
+			/* been converted, ignore */
+			goto next_block;
+		}
+		WARN_ON(imap.br_blockcount == 0);
+
+		/*
+		 * Note: while we initialise the firstblock/flist pair, they
+		 * should never be used because blocks should never be
+		 * allocated or freed for a delalloc extent and hence we need
+		 * don't cancel or finish them after the xfs_bunmapi() call.
+		 */
+		xfs_bmap_init(&flist, &firstblock);
+		error = xfs_bunmapi(NULL, ip, start_fsb, 1, 0, 1, &firstblock,
+					&flist, &done);
+		if (error)
+			break;
+
+		ASSERT(!flist.xbf_count && !flist.xbf_first);
+next_block:
+		start_fsb++;
+		remaining--;
+	} while(remaining > 0);
+
+	return error;
+}
diff --git a/fs/xfs/xfs_bmap.h b/fs/xfs/xfs_bmap.h
index 71ec9b6..3651191 100644
--- a/fs/xfs/xfs_bmap.h
+++ b/fs/xfs/xfs_bmap.h
@@ -394,6 +394,11 @@ xfs_bmap_count_blocks(
 	int			whichfork,
 	int			*count);
 
+int
+xfs_bmap_punch_delalloc_range(
+	struct xfs_inode	*ip,
+	xfs_fileoff_t		start_fsb,
+	xfs_fileoff_t		length);
 #endif	/* __KERNEL__ */
 
 #endif	/* __XFS_BMAP_H__ */
-- 
1.7.2.3

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[PATCH 2/2] xfs: push stale, pinned buffers on trylock failures

[Dave Chinner]

From: Dave Chinner <dchinner@redhat.com>

As reported by Nick Piggin, XFS is suffering from long pauses under
highly concurrent workloads when hosted on ramdisks. The problem is
that an inode buffer is stuck in the pinned state in memory and as a
result either the inode buffer or one of the inodes within the
buffer is stopping the tail of the log from being moved forward.

The system remains in this state until a periodic log force issued
by xfssyncd causes the buffer to be unpinned. The main problem is
that these are stale buffers, and are hence held locked until the
transaction/checkpoint that marked them state has been committed to
disk. When the filesystem gets into this state, only the xfssyncd
can cause the async transactions to be committed to disk and hence
unpin the inode buffer.

This problem was encountered when scaling the busy extent list, but
only the blocking lock interface was fixed to solve the problem.
Extend the same fix to the buffer trylock operations - if we fail to
lock a pinned, stale buffer, then force the log immediately so that
when the next attempt to lock it comes around, it will have been
unpinned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/linux-2.6/xfs_buf.c |   36 +++++++++++++++++++-----------------
 1 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index aa1d353..94e69a7 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -488,29 +488,21 @@ found:
 	spin_unlock(&pag->pag_buf_lock);
 	xfs_perag_put(pag);
 
-	/* Attempt to get the semaphore without sleeping,
-	 * if this does not work then we need to drop the
-	 * spinlock and do a hard attempt on the semaphore.
+	/*
+	 * Attempt to get the semaphore without sleeping first. if we fail then
+	 * do a blocking lock if requested.
 	 */
-	if (down_trylock(&bp->b_sema)) {
+	if (xfs_buf_cond_lock(bp)) {
 		if (!(flags & XBF_TRYLOCK)) {
 			/* wait for buffer ownership */
 			xfs_buf_lock(bp);
 			XFS_STATS_INC(xb_get_locked_waited);
 		} else {
-			/* We asked for a trylock and failed, no need
-			 * to look at file offset and length here, we
-			 * know that this buffer at least overlaps our
-			 * buffer and is locked, therefore our buffer
-			 * either does not exist, or is this buffer.
-			 */
+			/* We asked for a trylock and failed.  */
 			xfs_buf_rele(bp);
 			XFS_STATS_INC(xb_busy_locked);
 			return NULL;
 		}
-	} else {
-		/* trylock worked */
-		XB_SET_OWNER(bp);
 	}
 
 	if (bp->b_flags & XBF_STALE) {
@@ -876,10 +868,18 @@ xfs_buf_rele(
  */
 
 /*
- *	Locks a buffer object, if it is not already locked.
- *	Note that this in no way locks the underlying pages, so it is only
- *	useful for synchronizing concurrent use of buffer objects, not for
- *	synchronizing independent access to the underlying pages.
+ *	Locks a buffer object, if it is not already locked.  Note that this in
+ *	no way locks the underlying pages, so it is only useful for
+ *	synchronizing concurrent use of buffer objects, not for synchronizing
+ *	independent access to the underlying pages.
+ *
+ *	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_cond_lock(
@@ -890,6 +890,8 @@ xfs_buf_cond_lock(
 	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_cond_lock(bp, _RET_IP_);
 	return locked ? 0 : -EBUSY;
-- 
1.7.2.3

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Re: [PATCH 2/2] xfs: push stale, pinned buffers on trylock failures

[Christoph Hellwig]

> -	/* Attempt to get the semaphore without sleeping,
> -	 * if this does not work then we need to drop the
> -	 * spinlock and do a hard attempt on the semaphore.
> +	/*
> +	 * Attempt to get the semaphore without sleeping first. if we fail then
> +	 * do a blocking lock if requested.
>  	 */

You might as well remove the comment entirely, as it's utterly
pointless.

> -	if (down_trylock(&bp->b_sema)) {
> +	if (xfs_buf_cond_lock(bp)) {
>  		if (!(flags & XBF_TRYLOCK)) {
>  			/* wait for buffer ownership */
>  			xfs_buf_lock(bp);
>  			XFS_STATS_INC(xb_get_locked_waited);
>  		} else {
> -			/* We asked for a trylock and failed, no need
> -			 * to look at file offset and length here, we
> -			 * know that this buffer at least overlaps our
> -			 * buffer and is locked, therefore our buffer
> -			 * either does not exist, or is this buffer.
> -			 */
> +			/* We asked for a trylock and failed.  */
>  			xfs_buf_rele(bp);
>  			XFS_STATS_INC(xb_busy_locked);
>  			return NULL;
>  		}

In case we wait for the lock we now do the log force twice, but that
should be fine.

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Re: [PATCH 1/2] xfs: fix failed write truncation handling.

[Christoph Hellwig]

Looks good,


Reviewed-by: Christoph Hellwig <hch@lst.de>

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Re: [PATCH 2/2] xfs: push stale, pinned buffers on trylock failures

[Dave Chinner]

On Wed, Nov 24, 2010 at 12:20:51PM -0500, Christoph Hellwig wrote:
> > -	/* Attempt to get the semaphore without sleeping,
> > -	 * if this does not work then we need to drop the
> > -	 * spinlock and do a hard attempt on the semaphore.
> > +	/*
> > +	 * Attempt to get the semaphore without sleeping first. if we fail then
> > +	 * do a blocking lock if requested.
> >  	 */
> 
> You might as well remove the comment entirely, as it's utterly
> pointless.

Ok.

> > -	if (down_trylock(&bp->b_sema)) {
> > +	if (xfs_buf_cond_lock(bp)) {
> >  		if (!(flags & XBF_TRYLOCK)) {
> >  			/* wait for buffer ownership */
> >  			xfs_buf_lock(bp);
> >  			XFS_STATS_INC(xb_get_locked_waited);
> >  		} else {
> > -			/* We asked for a trylock and failed, no need
> > -			 * to look at file offset and length here, we
> > -			 * know that this buffer at least overlaps our
> > -			 * buffer and is locked, therefore our buffer
> > -			 * either does not exist, or is this buffer.
> > -			 */
> > +			/* We asked for a trylock and failed.  */
> >  			xfs_buf_rele(bp);
> >  			XFS_STATS_INC(xb_busy_locked);
> >  			return NULL;
> >  		}
> 
> In case we wait for the lock we now do the log force twice, but that
> should be fine.

Yeah, I considered ways of avoiding this, but when i actually
measured how often it occurs, it is quite rare because the log force
clears the entire backlog of pinned, stale buffers and so the log
force is lost in the noise of other IO. Hence I thought the more
robust method of fixing and calling xfs_buf_cond_lock() was a better
tradeoff.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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[PATCH v2] xfs: push stale, pinned buffers on trylock failures

[Dave Chinner]

From: Dave Chinner <dchinner@redhat.com>

As reported by Nick Piggin, XFS is suffering from long pauses under
highly concurrent workloads when hosted on ramdisks. The problem is
that an inode buffer is stuck in the pinned state in memory and as a
result either the inode buffer or one of the inodes within the
buffer is stopping the tail of the log from being moved forward.

The system remains in this state until a periodic log force issued
by xfssyncd causes the buffer to be unpinned. The main problem is
that these are stale buffers, and are hence held locked until the
transaction/checkpoint that marked them state has been committed to
disk. When the filesystem gets into this state, only the xfssyncd
can cause the async transactions to be committed to disk and hence
unpin the inode buffer.

This problem was encountered when scaling the busy extent list, but
only the blocking lock interface was fixed to solve the problem.
Extend the same fix to the buffer trylock operations - if we fail to
lock a pinned, stale buffer, then force the log immediately so that
when the next attempt to lock it comes around, it will have been
unpinned.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
Version 2
- fix up comments as per Christoph's recommendation.

 fs/xfs/linux-2.6/xfs_buf.c |   35 ++++++++++++++++-------------------
 1 files changed, 16 insertions(+), 19 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index aa1d353..4c5deb6 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -488,29 +488,16 @@ found:
 	spin_unlock(&pag->pag_buf_lock);
 	xfs_perag_put(pag);
 
-	/* Attempt to get the semaphore without sleeping,
-	 * if this does not work then we need to drop the
-	 * spinlock and do a hard attempt on the semaphore.
-	 */
-	if (down_trylock(&bp->b_sema)) {
+	if (xfs_buf_cond_lock(bp)) {
+		/* failed, so wait for the lock if requested. */
 		if (!(flags & XBF_TRYLOCK)) {
-			/* wait for buffer ownership */
 			xfs_buf_lock(bp);
 			XFS_STATS_INC(xb_get_locked_waited);
 		} else {
-			/* We asked for a trylock and failed, no need
-			 * to look at file offset and length here, we
-			 * know that this buffer at least overlaps our
-			 * buffer and is locked, therefore our buffer
-			 * either does not exist, or is this buffer.
-			 */
 			xfs_buf_rele(bp);
 			XFS_STATS_INC(xb_busy_locked);
 			return NULL;
 		}
-	} else {
-		/* trylock worked */
-		XB_SET_OWNER(bp);
 	}
 
 	if (bp->b_flags & XBF_STALE) {
@@ -876,10 +863,18 @@ xfs_buf_rele(
  */
 
 /*
- *	Locks a buffer object, if it is not already locked.
- *	Note that this in no way locks the underlying pages, so it is only
- *	useful for synchronizing concurrent use of buffer objects, not for
- *	synchronizing independent access to the underlying pages.
+ *	Locks a buffer object, if it is not already locked.  Note that this in
+ *	no way locks the underlying pages, so it is only useful for
+ *	synchronizing concurrent use of buffer objects, not for synchronizing
+ *	independent access to the underlying pages.
+ *
+ *	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_cond_lock(
@@ -890,6 +885,8 @@ xfs_buf_cond_lock(
 	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_cond_lock(bp, _RET_IP_);
 	return locked ? 0 : -EBUSY;

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Re: [PATCH v2] xfs: push stale, pinned buffers on trylock failures

[Christoph Hellwig]

Looks good,


Reviewed-by: Christoph Hellwig <hch@lst.de>

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