Re: [PATCH 3/5] btrfs: only take normal tickets into account in may_commit_transaction

[Nikolay Borisov <nborisov@suse.com>]

On 9.03.20 г. 22:23 ч., Josef Bacik wrote:
> In debugging a generic/320 failure on ppc64, Nikolay noticed that
> sometimes we'd ENOSPC out with plenty of space to reclaim if we had
> committed the transaction.  He further discovered that this was because
> there was a priority ticket that was small enough to fit in the free
> space currently in the space_info.  While that is a problem by itself,
> it exposed another flaw, that we consider priority tickets in
> may_commit_transaction.
> 
> Priority tickets are not allowed to commit the transaction, thus we
> shouldn't even consider them in may_commit_transaction.  Instead we need
> to only consider current normal tickets.  With this fix in place, we
> will properly commit the transaction.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/space-info.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
> index 8d00a9ee9458..d198cfd45cf7 100644
> --- a/fs/btrfs/space-info.c
> +++ b/fs/btrfs/space-info.c
> @@ -592,10 +592,7 @@ static int may_commit_transaction(struct btrfs_fs_info *fs_info,
>  	else
>  		cur_free_bytes = 0;
>  
> -	if (!list_empty(&space_info->priority_tickets))
> -		ticket = list_first_entry(&space_info->priority_tickets,
> -					  struct reserve_ticket, list);
> -	else if (!list_empty(&space_info->tickets))
> +	if (!list_empty(&space_info->tickets))
>  		ticket = list_first_entry(&space_info->tickets,
>  					  struct reserve_ticket, list);

I took another look at handle_reserve_ticket and in the case of
BTRFS_RESERVE_FLUSH_EVICT  which is also handled by the priority list we
simply ignore the prio ticket, is this correct at all?
evict_flush_states does in fact contain COMMIT_TRANS state?

>  	bytes_needed = (ticket) ? ticket->bytes : 0;
>