From: "Darrick J. Wong" djwong@kernel.org
commit 72a048c1056a72e37ea2ee34cc73d8c6d6cb4290 upstream.
While prototyping a free space defragmentation tool, I observed an unexpected IO error while running a sequence of commands that can be recreated by the following sequence of commands:
$ xfs_io -f -c "pwrite -S 0x58 -b 10m 0 10m" file1 $ cp --reflink=always file1 file2 $ punch-alternating -o 1 file2 $ xfs_io -c "funshare 0 10m" file2 fallocate: Input/output error
I then scraped this (abbreviated) stack trace from dmesg:
WARNING: CPU: 0 PID: 30788 at fs/iomap/buffered-io.c:577 iomap_write_begin+0x376/0x450 CPU: 0 PID: 30788 Comm: xfs_io Not tainted 5.14.0-rc6-xfsx #rc6 5ef57b62a900814b3e4d885c755e9014541c8732 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014 RIP: 0010:iomap_write_begin+0x376/0x450 RSP: 0018:ffffc90000c0fc20 EFLAGS: 00010297 RAX: 0000000000000001 RBX: ffffc90000c0fd10 RCX: 0000000000001000 RDX: ffffc90000c0fc54 RSI: 000000000000000c RDI: 000000000000000c RBP: ffff888005d5dbd8 R08: 0000000000102000 R09: ffffc90000c0fc50 R10: 0000000000b00000 R11: 0000000000101000 R12: ffffea0000336c40 R13: 0000000000001000 R14: ffffc90000c0fd10 R15: 0000000000101000 FS: 00007f4b8f62fe40(0000) GS:ffff88803ec00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000056361c554108 CR3: 000000000524e004 CR4: 00000000001706f0 Call Trace: iomap_unshare_actor+0x95/0x140 iomap_apply+0xfa/0x300 iomap_file_unshare+0x44/0x60 xfs_reflink_unshare+0x50/0x140 [xfs 61947ea9b3a73e79d747dbc1b90205e7987e4195] xfs_file_fallocate+0x27c/0x610 [xfs 61947ea9b3a73e79d747dbc1b90205e7987e4195] vfs_fallocate+0x133/0x330 __x64_sys_fallocate+0x3e/0x70 do_syscall_64+0x35/0x80 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f4b8f79140a
Looking at the iomap tracepoints, I saw this:
iomap_iter: dev 8:64 ino 0x100 pos 0 length 0 flags WRITE|0x80 (0x81) ops xfs_buffered_write_iomap_ops caller iomap_file_unshare iomap_iter_dstmap: dev 8:64 ino 0x100 bdev 8:64 addr -1 offset 0 length 131072 type DELALLOC flags SHARED iomap_iter_srcmap: dev 8:64 ino 0x100 bdev 8:64 addr 147456 offset 0 length 4096 type MAPPED flags iomap_iter: dev 8:64 ino 0x100 pos 0 length 4096 flags WRITE|0x80 (0x81) ops xfs_buffered_write_iomap_ops caller iomap_file_unshare iomap_iter_dstmap: dev 8:64 ino 0x100 bdev 8:64 addr -1 offset 4096 length 4096 type DELALLOC flags SHARED console: WARNING: CPU: 0 PID: 30788 at fs/iomap/buffered-io.c:577 iomap_write_begin+0x376/0x450
The first time funshare calls ->iomap_begin, xfs sees that the first block is shared and creates a 128k delalloc reservation in the COW fork. The delalloc reservation is returned as dstmap, and the shared block is returned as srcmap. So far so good.
funshare calls ->iomap_begin to try the second block. This time there's no srcmap (punch-alternating punched it out!) but we still have the delalloc reservation in the COW fork. Therefore, we again return the reservation as dstmap and the hole as srcmap. iomap_unshare_iter incorrectly tries to unshare the hole, which __iomap_write_begin rejects because shared regions must be fully written and therefore cannot require zeroing.
Therefore, change the buffered write iomap_begin function not to set IOMAP_F_SHARED when there isn't a source mapping to read from for the unsharing.
Signed-off-by: Darrick J. Wong djwong@kernel.org Reviewed-by: Christoph Hellwig hch@lst.de Reviewed-by: Chandan Babu R chandanrlinux@gmail.com Signed-off-by: Amir Goldstein amir73il@gmail.com Acked-by: Darrick J. Wong djwong@kernel.org --- fs/xfs/xfs_iomap.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c index 74bc2beadc23..bd5a25f4952d 100644 --- a/fs/xfs/xfs_iomap.c +++ b/fs/xfs/xfs_iomap.c @@ -1062,11 +1062,11 @@ xfs_buffered_write_iomap_begin( error = xfs_bmbt_to_iomap(ip, srcmap, &imap, 0); if (error) return error; - } else { - xfs_trim_extent(&cmap, offset_fsb, - imap.br_startoff - offset_fsb); + return xfs_bmbt_to_iomap(ip, iomap, &cmap, IOMAP_F_SHARED); } - return xfs_bmbt_to_iomap(ip, iomap, &cmap, IOMAP_F_SHARED); + + xfs_trim_extent(&cmap, offset_fsb, imap.br_startoff - offset_fsb); + return xfs_bmbt_to_iomap(ip, iomap, &cmap, 0);
out_unlock: xfs_iunlock(ip, XFS_ILOCK_EXCL);