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

[Josef Bacik <josef@toxicpanda.com>]

On 3/10/20 6:32 AM, Nikolay Borisov wrote:
> 
> 
> 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.

I'd have to put it both in there and priority_reclaim_data_space, this is 
cleaner.  Thanks,

Josef