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

#syz upstream

On Fri, Jul 24, 2026 at 10:24:30PM +0000, syzbot wrote:
> 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: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=021d10c4d4edc87daa03
> Link: https://syzkaller.appspot.com/ai_job?id=ca6bd6c4-fbcf-4533-af07-79b4a0eba74d
> To: "Chris Mason" <[email protected]>
> To: "David Sterba" <[email protected]>
> To: <[email protected]>
> To: "Yan, Zheng" <[email protected]>
> Cc: <[email protected]>
> 
> ---
> 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/[email protected]/T/
> 
> v1:
> https://lore.kernel.org/all/[email protected]/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
> -- 
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