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

Josef Bacik
2020-03-13T19:54:36Z

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