Re: [PATCH 5/5] btrfs: run btrfs_try_granting_tickets if a priority ticket fails

Nikolay Borisov
2020-03-10T10:32:54Z

On 9.03.20 г. 22:23 ч., Josef Bacik wrote:
> With normal tickets we could have a large reservation at the front of
> the list that is unable to be satisfied, but a smaller ticket later on
> that can be satisfied. The way we handle this is to run
> btrfs_try_granting_tickets() in maybe_fail_all_tickets().
>
> However no such protection exists for priority tickets. Fix this by
> handling it in handle_reserve_ticket(). If we've returned after
> attempting to flush space in a priority related way, we'll still be on
> the priority list and need to be removed.
>
> We rely on the flushing to free up space and wake the ticket, but if
> there is not enough space to reclaim _but_ there's enough space in the
> space_info to handle subsequent reservations then we would have gotten
> an ENOSPC erroneously.
>
> Address this by catching where we are still on the list, meaning we were
> a priority ticket, and removing ourselves and then running
> btrfs_try_granting_tickets(). This will handle this particular corner
> case.
>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
> fs/btrfs/space-info.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index 77ea204f0b6a..03172ecd9c0b 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -1256,11 +1256,17 @@ static int handle_reserve_ticket(struct btrfs_fs_info *fs_info,
> ret = ticket->error;
> if (ticket->bytes || ticket->error) {
> /*
> - * Need to delete here for priority tickets. For regular tickets
> - * either the async reclaim job deletes the ticket from the list
> - * or we delete it ourselves at wait_reserve_ticket().
> + * We were a priority ticket, so we need to delete ourselves
> + * from the list. Because we could have other priority tickets
> + * behind us that require less space, run
> + * btrfs_try_granting_tickets() to see if their reservations can
> + * now be made.
> */
> - list_del_init(&ticket->list);
> + if (!list_empty(&ticket->list)) {
> + list_del_init(&ticket->list);
> + btrfs_try_granting_tickets(fs_info, space_info);
> + }

I'd rather have this handled in priority_reclaim_metadata_space.
> +
> if (!ret)
> ret = -ENOSPC;
> }
>