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

[Nikolay Borisov <nborisov@suse.com>]

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;
>  	}
>