Re: [PATCH 06/18] btrfs: flush reservations during quota disable

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Jul 05, 2023 at 04:20:43PM -0700, Boris Burkov wrote:
> The following sequence:
> enable simple quotas
> do some writes
>     reserve space
>     create ordered_extent
>         release rsv (store rsv_bytes in OE, mark QGROUP_RESERVED bits)
> disable quotas
> enable simple quotas
>     set qgroup rsv to 0 on all subvols
> ordered_extent finishes
>     create delayed ref with rsv_bytes from before
> run delayed ref
>     record_simple_quota_delta
>         free rsv_bytes (0 -> -rsv_delta)
> 
> results in us reliably underflowing the subvolume's qgroup rsv counter,
> because disabling/re-enabling quotas toggles reservation counters down
> to 0, but does not remove other file system state which represents
> successful acquisition of qgroup rsv space. Specifically metadata rsv
> counters on the root object and rsv_bytes on ordered_extent objects that
> have released their reservation as well as the corresponding
> QGROUP_RESERVED extent bits.
> 
> Normal qgroups gets away with this, I believe because it forces more
> work to happen on transaction commit, but I am not certain it is totally
> safe from the ordered_extent/leaked extent bit variant. Simple quotas
> hits this reliably.
> 
> The intent of the fix is to make disable take the time to clear that
> external to qgroups state as well: after flipping off the quota bit on
> fs_info, flush delalloc and ordered extents, clearing the extent bits
> along the way. This makes it so there are no ordered extents or meta
> prealloc hanging around from the first enablement period during the second.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef