From: Brian Foster bfoster@redhat.com
commit 4390f019ad7866c3791c3d768d2ff185d89e8ebe upstream.
fallocate unshare mode explicitly breaks extent sharing. When a command completes, it checks the data fork for any remaining shared extents to determine whether the reflink inode flag and COW fork preallocation can be removed. This logic doesn't consider in-core pagecache and I/O state, however, which means we can unsafely remove COW fork blocks that are still needed under certain conditions.
For example, consider the following command sequence:
xfs_io -fc "pwrite 0 1k" -c "reflink <file> 0 256k 1k" \ -c "pwrite 0 32k" -c "funshare 0 1k" <file>
This allocates a data block at offset 0, shares it, and then overwrites it with a larger buffered write. The overwrite triggers COW fork preallocation, 32 blocks by default, which maps the entire 32k write to delalloc in the COW fork. All but the shared block at offset 0 remains hole mapped in the data fork. The unshare command redirties and flushes the folio at offset 0, removing the only shared extent from the inode. Since the inode no longer maps shared extents, unshare purges the COW fork before the remaining 28k may have written back.
This leaves dirty pagecache backed by holes, which writeback quietly skips, thus leaving clean, non-zeroed pagecache over holes in the file. To verify, fiemap shows holes in the first 32k of the file and reads return different data across a remount:
$ xfs_io -c "fiemap -v" <file> <file>: EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS ... 1: [8..511]: hole 504 ... $ xfs_io -c "pread -v 4k 8" <file> 00001000: cd cd cd cd cd cd cd cd ........ $ umount <mnt>; mount <dev> <mnt> $ xfs_io -c "pread -v 4k 8" <file> 00001000: 00 00 00 00 00 00 00 00 ........
To avoid this problem, make unshare follow the same rules used for background cowblock scanning and never purge the COW fork for inodes with dirty pagecache or in-flight I/O.
Fixes: 46afb0628b86347 ("xfs: only flush the unshared range in xfs_reflink_unshare") Signed-off-by: Brian Foster bfoster@redhat.com Reviewed-by: Darrick J. Wong djwong@kernel.org Signed-off-by: Carlos Maiolino cem@kernel.org Signed-off-by: Catherine Hoang catherine.hoang@oracle.com Acked-by: Darrick J. Wong djwong@kernel.org --- fs/xfs/xfs_icache.c | 8 +------- fs/xfs/xfs_reflink.c | 3 +++ fs/xfs/xfs_reflink.h | 19 +++++++++++++++++++ 3 files changed, 23 insertions(+), 7 deletions(-)
diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c index 63304154006d..c54a7c60e063 100644 --- a/fs/xfs/xfs_icache.c +++ b/fs/xfs/xfs_icache.c @@ -1271,13 +1271,7 @@ xfs_prep_free_cowblocks( */ if (!sync && inode_is_open_for_write(VFS_I(ip))) return false; - if ((VFS_I(ip)->i_state & I_DIRTY_PAGES) || - mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_DIRTY) || - mapping_tagged(VFS_I(ip)->i_mapping, PAGECACHE_TAG_WRITEBACK) || - atomic_read(&VFS_I(ip)->i_dio_count)) - return false; - - return true; + return xfs_can_free_cowblocks(ip); }
/* diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c index 3431d0d8b6f3..4058cf361d21 100644 --- a/fs/xfs/xfs_reflink.c +++ b/fs/xfs/xfs_reflink.c @@ -1600,6 +1600,9 @@ xfs_reflink_clear_inode_flag(
ASSERT(xfs_is_reflink_inode(ip));
+ if (!xfs_can_free_cowblocks(ip)) + return 0; + error = xfs_reflink_inode_has_shared_extents(*tpp, ip, &needs_flag); if (error || needs_flag) return error; diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h index 65c5dfe17ecf..67a335b247b1 100644 --- a/fs/xfs/xfs_reflink.h +++ b/fs/xfs/xfs_reflink.h @@ -16,6 +16,25 @@ static inline bool xfs_is_cow_inode(struct xfs_inode *ip) return xfs_is_reflink_inode(ip) || xfs_is_always_cow_inode(ip); }
+/* + * Check whether it is safe to free COW fork blocks from an inode. It is unsafe + * to do so when an inode has dirty cache or I/O in-flight, even if no shared + * extents exist in the data fork, because outstanding I/O may target blocks + * that were speculatively allocated to the COW fork. + */ +static inline bool +xfs_can_free_cowblocks(struct xfs_inode *ip) +{ + struct inode *inode = VFS_I(ip); + + if ((inode->i_state & I_DIRTY_PAGES) || + mapping_tagged(inode->i_mapping, PAGECACHE_TAG_DIRTY) || + mapping_tagged(inode->i_mapping, PAGECACHE_TAG_WRITEBACK) || + atomic_read(&inode->i_dio_count)) + return false; + return true; +} + extern int xfs_reflink_trim_around_shared(struct xfs_inode *ip, struct xfs_bmbt_irec *irec, bool *shared); int xfs_bmap_trim_cow(struct xfs_inode *ip, struct xfs_bmbt_irec *imap,