[PATCH RFC v3] btrfs: commit current transaction in btrfs_relocate_block_group

["syzbot" <syzbot@kernel.org>]

During block group relocation, btrfs_relocate_block_group() is called to
relocate a block group. It first makes the block group read-only by calling
btrfs_inc_block_group_ro(). If there are pending space reservations or
pinned extents in the current transaction, making the block group read-only
can fail, leading to a transaction abort and a warning in
cleanup_transaction() during the transaction commit in
prepare_to_relocate().

To fix this, commit the current transaction before calling
btrfs_inc_block_group_ro() in btrfs_relocate_block_group(). This drains the
current transaction before the block group becomes read-only, ensuring that
pending extents are unpinned and space reservations are committed. This
does not claim global settlement of every reservation or later transaction,
but it provides a clean state for marking the block group read-only. The
setup commit in prepare_to_relocate() remains necessary because it commits
the transaction started during the relocation setup (such as creating the
reloc tree) to ensure everything is in a consistent state before starting
to move extents.

WARNING: fs/btrfs/transaction.c:2068 at cleanup_transaction+0x727/0x7c0
Call Trace:
 <TASK>
 btrfs_commit_transaction+0x262c/0x30b0 fs/btrfs/transaction.c:2664
 prepare_to_relocate+0x3dd/0x4e0 fs/btrfs/relocation.c:3541
 relocate_block_group+0x141/0xe90 fs/btrfs/relocation.c:3566
 do_nonremap_reloc+0xa7/0x560 fs/btrfs/relocation.c:5323
 btrfs_relocate_block_group+0x6e2/0xaf0 fs/btrfs/relocation.c:5490
 btrfs_relocate_chunk+0x114/0x830 fs/btrfs/volumes.c:3647
 __btrfs_balance+0x1b6e/0x29e0 fs/btrfs/volumes.c:4586
 btrfs_balance+0xaa6/0x1180 fs/btrfs/volumes.c:4973
 btrfs_ioctl_balance+0x3dd/0x640 fs/btrfs/ioctl.c:3474
 </TASK>

Fixes: 3fd0a5585eb9 ("Btrfs: Metadata ENOSPC handling for balance")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+021d10c4d4edc87daa03@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=021d10c4d4edc87daa03
Link: https://syzkaller.appspot.com/ai_job?id=ca6bd6c4-fbcf-4533-af07-79b4a0eba74d
To: "Chris Mason" <clm@fb.com>
To: "David Sterba" <dsterba@suse.com>
To: <linux-btrfs@vger.kernel.org>
To: "Yan, Zheng" <zheng.yan@oracle.com>
Cc: <linux-kernel@vger.kernel.org>

---
v3:
- Updated the commit description to clarify that the new call drains the current transaction before the block group becomes read-only.
- Clarified that this does not claim global settlement of every reservation or later transaction.
- Explained why the setup commit in prepare_to_relocate() remains necessary.

v2:
- Replaced the transaction abort changes in the commit path with committing the current transaction before making the block group read-only during relocation.
- Removed changes to fs/btrfs/qgroup.c, fs/btrfs/root-tree.c, fs/btrfs/transaction.c, and fs/btrfs/transaction.h.
- Added a call to btrfs_commit_current_transaction() in btrfs_relocate_block_group() before calling btrfs_inc_block_group_ro().
https://lore.kernel.org/all/38fe7b3f-75f7-47c9-9554-db7628338b20@mail.kernel.org/T/

v1:
https://lore.kernel.org/all/1586eaf3-2208-42eb-8877-7c4f83a6638c@mail.kernel.org/T/
---
diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index fb85bc8b3..05f5110c2 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -5430,6 +5430,10 @@ int btrfs_relocate_block_group(struct btrfs_fs_info *fs_info, u64 group_start,
 	if (ret < 0)
 		goto out_put_rc;
 
+	ret = btrfs_commit_current_transaction(extent_root);
+	if (ret)
+		goto out;
+
 	ret = btrfs_inc_block_group_ro(rc->block_group, true);
 	if (ret)
 		goto out;


base-commit: 8cdeaa50eae8dad34885515f62559ee83e7e8dda
-- 
This is an AI-generated patch subject to moderation.
Reply with '#syz upstream' to Sign-off the patch as a human author
and send it to the upstream kernel mailing lists.
Reply with '#syz reject' to reject it ('#syz unreject' to undo).

See https://goo.gle/syzbot-ai-patches for information about AI-generated patches.
You can comment on the patch as usual, syzbot will try to address
the comments and send a new version of the patch if necessary.
syzbot engineers can be reached at syzkaller@googlegroups.com.