Re: [PATCH v5 10/18] btrfs: track original extent owner in head_ref

[Josef Bacik <josef@toxicpanda.com>]

On Thu, Jul 27, 2023 at 03:12:57PM -0700, Boris Burkov wrote:
> Simple quotas requires tracking the original creating root of any given
> extent. This gets complicated when multiple subvolumes create
> overlapping/contradictory refs in the same transaction. For example,
> due to modifying or deleting an extent while also snapshotting it.
> 
> To resolve this in a general way, take advantage of the fact that we are
> essentially already tracking this for handling releasing reservations.
> The head ref coalesces the various refs and uses must_insert_reserved to
> check if it needs to create an extent/free reservation. Store the ref
> that set must_insert_reserved as the owning ref on the head ref.
> 
> Note that this can result in writing an extent for the very first time
> with an owner different from its only ref, but it will look the same as
> if you first created it with the original owning ref, then added the
> other ref, then removed the owning ref.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>

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

Thanks,

Josef