linux-kernel - Re: [PATCH 2/2] FUSE: fix congested state leak on aborted connections

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Date:   Wed, 30 May 2018 16:22:02 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Jan Kara <jack@...e.cz>
Cc:     Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Joshua Miller <joshmiller@...com>,
        kernel-team <kernel-team@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        stable <stable@...r.kernel.org>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/2] FUSE: fix congested state leak on aborted connections

On Tue, Feb 6, 2018 at 5:25 PM, Jan Kara <jack@...e.cz> wrote:
> On Fri 02-02-18 09:54:14, Tejun Heo wrote:
>> If a connection gets aborted while congested, FUSE can leave
>> nr_wb_congested[] stuck until reboot causing wait_iff_congested() to
>> wait spuriously which can lead to severe performance degradation.
>>
>> The leak is caused by gating congestion state clearing with
>> fc->connected test in request_end().  This was added way back in 2009
>> by 26c3679101db ("fuse: destroy bdi on umount").  While the commit
>> description doesn't explain why the test was added, it most likely was
>> to avoid dereferencing bdi after it got destroyed.
>>
>> Since then, bdi lifetime rules have changed many times and now we're
>> always guaranteed to have access to the bdi while the superblock is
>> alive (fc->sb).
>>
>> Drop fc->connected conditional to avoid leaking congestion states.
>>
>> Signed-off-by: Tejun Heo <tj@...nel.org>
>> Reported-by: Joshua Miller <joshmiller@...com>
>> Cc: Johannes Weiner <hannes@...xchg.org>
>> Cc: Miklos Szeredi <miklos@...redi.hu>
>> Cc: Jan Kara <jack@...e.cz>
>> Cc: stable@...r.kernel.org
>
> Yeah, this should be fine AFAICT but my knowledge of FUSE is very cursory.
> Anyway:
>
> Acked-by: Jan Kara <jack@...e.cz>

Can't say I fully understand how the global "is any bdi congested"
state is used in direct reclaim, but the patch is an obvious
improvement, so applied.

Thanks,
Miklos