Re: [PATCH 3/4] btrfs: clamp to avoid squota underflow

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2026/5/13 05:04, Boris Burkov 写道:
> Simple quota accounting can undercount metadata tree block allocations
> in certain scenarios. When an undercounted subvolume is deleted and its
> tree blocks freed, the free deltas decrement rfer/excl past zero,
> wrapping the u64 to a value near U64_MAX.
> 
> Once wrapped, can_delete_squota_qgroup() sees non-zero rfer and refuses
> to delete the qgroup. The qgroup becomes permanently orphaned in the
> quota tree, since there is no subvolume left to generate frees that
> would bring the counter back to zero.
> 
> While we ultimately want to fix any mis-accounting at the source, it is
> also helpful and worthwhile to mitigate the damage by clamping rfer and
> excl to zero on underflow rather than allowing the u64 to wrap. This at
> least allows us to clean up the messed up qgroups on subvol deletion.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>
> ---
>   fs/btrfs/qgroup.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
> index 96746ca19e17..a93d49015fc7 100644
> --- a/fs/btrfs/qgroup.c
> +++ b/fs/btrfs/qgroup.c
> @@ -4968,8 +4968,19 @@ int btrfs_record_squota_delta(struct btrfs_fs_info *fs_info,
>   	list_for_each_entry(qg, &qgroup_list, iterator) {
>   		struct btrfs_qgroup_list *glist;
>   
> -		qg->excl += num_bytes * sign;
> -		qg->rfer += num_bytes * sign;
> +		ASSERT(qg->excl == qg->rfer);
> +		if (sign < 0 && qg->excl < num_bytes) {
> +			btrfs_warn(fs_info,
> +				   "squota underflow qg %hu/%llu excl %llu num_bytes %llu",
> +				   btrfs_qgroup_level(qg->qgroupid),
> +				   btrfs_qgroup_subvolid(qg->qgroupid),
> +				   qg->excl, num_bytes);

This may not be noisy enough for fstests to catch.

Maybe a WARN_ON_ONCE()?

Thanks,
Qu

> +			qg->excl = 0;
> +			qg->rfer = 0;
> +		} else {
> +			qg->excl += num_bytes * sign;
> +			qg->rfer += num_bytes * sign;
> +		}
>   		qgroup_dirty(fs_info, qg);
>   
>   		list_for_each_entry(glist, &qg->groups, next_group)