[PATCH 5.10 v2 2/3] xfs: only set IOMAP_F_SHARED when providing a srcmap to a write

Amir Goldstein
2022-08-10T14:16:05Z

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);
--
2.25.1