xfs: validate writeback mapping using data fork seq counter

The writeback code caches the current extent mapping across multiple
xfs_do_writepage() calls to avoid repeated lookups for sequential
pages backed by the same extent. This is known to be slightly racy
with extent fork changes in certain difficult to reproduce
scenarios. The cached extent is trimmed to within EOF to help avoid
the most common vector for this problem via speculative
preallocation management, but this is a band-aid that does not
address the fundamental problem.

Now that we have an xfs_ifork sequence counter mechanism used to
facilitate COW writeback, we can use the same mechanism to validate
consistency between the data fork and cached writeback mappings. On
its face, this is somewhat of a big hammer approach because any
change to the data fork invalidates any mapping currently cached by
a writeback in progress regardless of whether the data fork change
overlaps with the range under writeback. In practice, however, the
impact of this approach is minimal in most cases.

First, data fork changes (delayed allocations) caused by sustained
sequential buffered writes are amortized across speculative
preallocations. This means that a cached mapping won't be
invalidated by each buffered write of a common file copy workload,
but rather only on less frequent allocation events. Second, the
extent tree is always entirely in-core so an additional lookup of a
usable extent mostly costs a shared ilock cycle and in-memory tree
lookup. This means that a cached mapping reval is relatively cheap
compared to the I/O itself. Third, spurious invalidations don't
impact ioend construction. This means that even if the same extent
is revalidated multiple times across multiple writepage instances,
we still construct and submit the same size ioend (and bio) if the
blocks are physically contiguous.

Update struct xfs_writepage_ctx with a new field to hold the
sequence number of the data fork associated with the currently
cached mapping. Check the wpc seqno against the data fork when the
mapping is validated and reestablish the mapping whenever the fork
has changed since the mapping was cached. This ensures that
writeback always uses a valid extent mapping and thus prevents lost
writebacks and stale delalloc block problems.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

2 files changed, 49 insertions, 16 deletions

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index d9048bcea49c..5b0256a8a420 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -29,6 +29,7 @@
 struct xfs_writepage_ctx {
 	struct xfs_bmbt_irec    imap;
 	unsigned int		io_type;
+	unsigned int		data_seq;
 	unsigned int		cow_seq;
 	struct xfs_ioend	*ioend;
 };
@@ -301,6 +302,42 @@ xfs_end_bio(
 		xfs_destroy_ioend(ioend, blk_status_to_errno(bio->bi_status));
 }
 
+/*
+ * Fast revalidation of the cached writeback mapping. Return true if the current
+ * mapping is valid, false otherwise.
+ */
+static bool
+xfs_imap_valid(
+	struct xfs_writepage_ctx	*wpc,
+	struct xfs_inode		*ip,
+	xfs_fileoff_t			offset_fsb)
+{
+	if (offset_fsb < wpc->imap.br_startoff ||
+	    offset_fsb >= wpc->imap.br_startoff + wpc->imap.br_blockcount)
+		return false;
+	/*
+	 * If this is a COW mapping, it is sufficient to check that the mapping
+	 * covers the offset. Be careful to check this first because the caller
+	 * can revalidate a COW mapping without updating the data seqno.
+	 */
+	if (wpc->io_type == XFS_IO_COW)
+		return true;
+
+	/*
+	 * This is not a COW mapping. Check the sequence number of the data fork
+	 * because concurrent changes could have invalidated the extent. Check
+	 * the COW fork because concurrent changes since the last time we
+	 * checked (and found nothing at this offset) could have added
+	 * overlapping blocks.
+	 */
+	if (wpc->data_seq != READ_ONCE(ip->i_df.if_seq))
+		return false;
+	if (xfs_inode_has_cow_data(ip) &&
+	    wpc->cow_seq != READ_ONCE(ip->i_cowfp->if_seq))
+		return false;
+	return true;
+}
+
 STATIC int
 xfs_map_blocks(
 	struct xfs_writepage_ctx *wpc,
@@ -315,9 +352,11 @@ xfs_map_blocks(
 	struct xfs_bmbt_irec	imap;
 	int			whichfork = XFS_DATA_FORK;
 	struct xfs_iext_cursor	icur;
-	bool			imap_valid;
 	int			error = 0;
 
+	if (XFS_FORCED_SHUTDOWN(mp))
+		return -EIO;
+
 	/*
 	 * We have to make sure the cached mapping is within EOF to protect
 	 * against eofblocks trimming on file release leaving us with a stale
@@ -346,17 +385,9 @@ xfs_map_blocks(
 	 * against concurrent updates and provides a memory barrier on the way
 	 * out that ensures that we always see the current value.
 	 */
-	imap_valid = offset_fsb >= wpc->imap.br_startoff &&
-		     offset_fsb < wpc->imap.br_startoff + wpc->imap.br_blockcount;
-	if (imap_valid &&
-	    (!xfs_inode_has_cow_data(ip) ||
-	     wpc->io_type == XFS_IO_COW ||
-	     wpc->cow_seq == READ_ONCE(ip->i_cowfp->if_seq)))
+	if (xfs_imap_valid(wpc, ip, offset_fsb))
 		return 0;
 
-	if (XFS_FORCED_SHUTDOWN(mp))
-		return -EIO;
-
 	/*
 	 * If we don't have a valid map, now it's time to get a new one for this
 	 * offset.  This will convert delayed allocations (including COW ones)
@@ -403,9 +434,10 @@ xfs_map_blocks(
 	}
 
 	/*
-	 * Map valid and no COW extent in the way?  We're done.
+	 * No COW extent overlap. Revalidate now that we may have updated
+	 * ->cow_seq. If the data mapping is still valid, we're done.
 	 */
-	if (imap_valid) {
+	if (xfs_imap_valid(wpc, ip, offset_fsb)) {
 		xfs_iunlock(ip, XFS_ILOCK_SHARED);
 		return 0;
 	}
@@ -417,6 +449,7 @@ xfs_map_blocks(
 	 */
 	if (!xfs_iext_lookup_extent(ip, &ip->i_df, offset_fsb, &icur, &imap))
 		imap.br_startoff = end_fsb;	/* fake a hole past EOF */
+	wpc->data_seq = READ_ONCE(ip->i_df.if_seq);
 	xfs_iunlock(ip, XFS_ILOCK_SHARED);
 
 	if (imap.br_startoff > offset_fsb) {
@@ -454,7 +487,8 @@ xfs_map_blocks(
 	return 0;
 allocate_blocks:
 	error = xfs_iomap_write_allocate(ip, whichfork, offset, &imap,
-			&wpc->cow_seq);
+			whichfork == XFS_COW_FORK ?
+					 &wpc->cow_seq : &wpc->data_seq);
 	if (error)
 		return error;
 	ASSERT(whichfork == XFS_COW_FORK || cow_fsb == NULLFILEOFF ||
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 27c93b5f029d..ab69caa685b4 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -681,7 +681,7 @@ xfs_iomap_write_allocate(
 	int		whichfork,
 	xfs_off_t	offset,
 	xfs_bmbt_irec_t *imap,
-	unsigned int	*cow_seq)
+	unsigned int	*seq)
 {
 	xfs_mount_t	*mp = ip->i_mount;
 	struct xfs_ifork *ifp = XFS_IFORK_PTR(ip, whichfork);
@@ -797,8 +797,7 @@ xfs_iomap_write_allocate(
 			if (error)
 				goto error0;
 
-			if (whichfork == XFS_COW_FORK)
-				*cow_seq = READ_ONCE(ifp->if_seq);
+			*seq = READ_ONCE(ifp->if_seq);
 			xfs_iunlock(ip, XFS_ILOCK_EXCL);
 		}