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From: Nikolay Borisov <nborisov@suse.com>
To: Josef Bacik <josef@toxicpanda.com>,
	linux-btrfs@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 4/5] btrfs: only check priority tickets for priority flushing
Date: Tue, 17 Mar 2020 14:55:41 +0200	[thread overview]
Message-ID: <e314aa12-f4dc-27c6-a355-ce2b6404786c@suse.com> (raw)
In-Reply-To: <20200313195809.141753-5-josef@toxicpanda.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>

  reply	other threads:[~2020-03-17 12:55 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-13 19:58 [PATCH 0/5][v2] Deal with a few ENOSPC corner cases Josef Bacik
2020-03-13 19:58 ` [PATCH 1/5] btrfs: Improve global reserve stealing logic Josef Bacik
2020-03-17 12:46   ` Nikolay Borisov
2020-03-13 19:58 ` [PATCH 2/5] btrfs: allow us to use up to 90% of the global rsv for unlink Josef Bacik
2020-03-17 12:46   ` Nikolay Borisov
2020-03-13 19:58 ` [PATCH 3/5] btrfs: Account for trans_block_rsv in may_commit_transaction Josef Bacik
2020-03-13 19:58 ` [PATCH 4/5] btrfs: only check priority tickets for priority flushing Josef Bacik
2020-03-17 12:55   ` Nikolay Borisov [this message]
2020-03-13 19:58 ` [PATCH 5/5] btrfs: run btrfs_try_granting_tickets if a priority ticket fails Josef Bacik
2020-03-17 12:59   ` Nikolay Borisov
2020-03-17 15:46 ` [PATCH 0/5][v2] Deal with a few ENOSPC corner cases Nikolay Borisov
2020-03-25 15:50 ` David Sterba
2020-03-25 15:52   ` Nikolay Borisov
2020-03-25 18:33     ` David Sterba
2020-04-03 15:46 ` David Sterba
  -- strict thread matches above, loose matches on Subject: below --
2020-03-09 20:23 [PATCH 0/5] " Josef Bacik
2020-03-09 20:23 ` [PATCH 4/5] btrfs: only check priority tickets for priority flushing Josef Bacik
2020-03-10 10:30   ` Nikolay Borisov

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