From 7588cbeec6df925ef6142a7e48762896c06007a8 Mon Sep 17 00:00:00 2001 From: Christoph Hellwig Date: Fri, 15 Feb 2019 08:02:50 -0800 Subject: [PATCH] xfs: retry COW fork delalloc conversion when no extent was found 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. This means that for example we can have another thread doing a direct I/O completion that moves extents from the COW to the data fork race with writeback. While this race is very hard to hit the always_cow seems to reproduce it reasonably well, and it also exists without that. 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 Reviewed-by: Darrick J. Wong Signed-off-by: Darrick J. Wong --- 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 a6abb7125203..2ed8733eca49 100644 --- a/fs/xfs/xfs_aops.c +++ b/fs/xfs/xfs_aops.c @@ -334,7 +334,8 @@ xfs_imap_valid( * extent that maps offset_fsb in wpc->imap. * * 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( @@ -375,6 +376,7 @@ 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)) @@ -404,6 +406,7 @@ 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)); @@ -471,8 +474,19 @@ xfs_map_blocks( return 0; allocate_blocks: error = xfs_convert_blocks(wpc, ip, offset_fsb); - 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; + } /* * Due to merging the return real extent might be larger than the -- GitLab