[PATCH 5.10 CANDIDATE 3/4] xfs: only set IOMAP_F_SHARED when providing a srcmap to a write

[Amir Goldstein <amir73il@gmail.com>]

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>
---
 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