Re: [PATCH] btrfs: commit current transaction in btrfs_relocate_block_group

[Qu Wenruo <wqu@suse.com>]

在 2026/7/28 18:15, syzbot 写道:
> From: Bartosz Chronowski <immersa.bartosz.chronowski@gmail.com>
> 
> 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,

What if a another workload triggered space reservation between the just 
committed transaction and btrfs_inc_block_group_ro()?

> 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
> Signed-off-by: Bartosz Chronowski <immersa.bartosz.chronowski@gmail.com>
> 
> ---
> 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