[PATCH 11/11] xfs: retry COW fork delalloc conversion when no extent was found

[Christoph Hellwig <hch@lst.de>]

While we can only truncate a block under the page lock for the current
page, there is no high-level synchronization for moving extents from the
COW to the data fork.  Because of that there is a chance that a
delalloc conversion for the COW fork might not find any extents to
convert.  In that case we should retry the whole block lookup and now
find the blocks in the data fork.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_aops.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index d95156d8e801..d271a9b4c93a 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -338,7 +338,8 @@ xfs_imap_valid(
  * consistency with what the caller expects.
  *
  * The current page is held locked so nothing could have removed the block
- * backing offset_fsb.
+ * backing offset_fsb, although it could have moved from the COW to the data
+ * fork by another thread.
  */
 static int
 xfs_convert_blocks(
@@ -381,6 +382,7 @@ xfs_map_blocks(
 	xfs_fileoff_t		cow_fsb = NULLFILEOFF;
 	struct xfs_bmbt_irec	imap;
 	struct xfs_iext_cursor	icur;
+	int			retries = 0;
 	int			error = 0;
 
 	if (XFS_FORCED_SHUTDOWN(mp))
@@ -424,6 +426,7 @@ xfs_map_blocks(
 	 * into real extents.  If we return without a valid map, it means we
 	 * landed in a hole and we skip the block.
 	 */
+retry:
 	xfs_ilock(ip, XFS_ILOCK_SHARED);
 	ASSERT(ip->i_d.di_format != XFS_DINODE_FMT_BTREE ||
 	       (ip->i_df.if_flags & XFS_IFEXTENTS));
@@ -491,8 +494,19 @@ xfs_map_blocks(
 	return 0;
 allocate_blocks:
 	error = xfs_convert_blocks(wpc, ip, offset_fsb, &imap);
-	if (error)
+	if (error) {
+		/*
+		 * If we failed to find the extent in the COW fork we might have
+		 * raced with a COW to data fork conversion or truncate.
+		 * Restart the lookup to catch the extent in the data fork for
+		 * the former case, but prevent additional retries to avoid
+		 * looping forever for the latter case.
+		 */
+		if (error == -EAGAIN && wpc->fork == XFS_COW_FORK && !retries++)
+			goto retry;
+		ASSERT(error != -EAGAIN);
 		return error;
+	}
 	ASSERT(wpc->imap.br_startoff <= offset_fsb);
 	ASSERT(wpc->imap.br_startoff + wpc->imap.br_blockcount >= offset_fsb);
 	ASSERT(wpc->fork == XFS_COW_FORK || cow_fsb == NULLFILEOFF ||
-- 
2.20.1