Re: [PATCH 4/5] btrfs: only check priority tickets for priority flushing

[Nikolay Borisov <nborisov@suse.com>]

On 13.03.20 г. 21:58 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.
> 
> Consider the following scenario.  There is no more space to reclaim in
> the fs without committing the transaction.  Assume there's 1mib of space
> free in the space info, but there are pending normal tickets with 2mib
> reservations.
> 
> Now a priority ticket comes in with a .5mib reservation.  Because we
> have normal tickets pending we add ourselves to the priority list,
> despite the fact that we could satisfy this reservation.
> 
> The flushing machinery now gets to the point where it wants to commit
> the transaction, but because there's a .5mib ticket on the priority list
> and we have 1mib of free space we assume the ticket will be granted
> soon, so we bail without committing the transaction.
> 
> Meanwhile the priority flushing does not commit the transaction, and
> eventually fails with an ENOSPC.  Then all other tickets are failed with
> ENOSPC because we were never able to actually commit the transaction.
> 
> The fix for this is we should have simply granted the priority flusher
> his reservation, because there was space to make the reservation.
> Priority flushers by definition take priority, so they are allowed to
> make their reservations before any previous normal tickets.  By not
> adding this priority ticket to the list the normal flushing mechanisms
> will then commit the transaction and everything will continue normally.
> 
> We still need to serialize ourselves with other priority tickets, so if
> there are any tickets on the priority list then we need to add ourselves
> to that list in order to maintain the serialization between priority
> tickets.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>