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

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Date:   Tue, 6 Feb 2018 17:25:18 +0100
From:   Jan Kara <jack@...e.cz>
To:     Tejun Heo <tj@...nel.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Miklos Szeredi <miklos@...redi.hu>,
        Joshua Miller <joshmiller@...com>, kernel-team@...com,
        Johannes Weiner <hannes@...xchg.org>, Jan Kara <jack@...e.cz>,
        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 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>

								Honza

> ---
>  fs/fuse/dev.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -381,8 +381,7 @@ static void request_end(struct fuse_conn
>  		if (!fc->blocked && waitqueue_active(&fc->blocked_waitq))
>  			wake_up(&fc->blocked_waitq);
>  
> -		if (fc->num_background == fc->congestion_threshold &&
> -		    fc->connected && fc->sb) {
> +		if (fc->num_background == fc->congestion_threshold && fc->sb) {
>  			clear_bdi_congested(fc->sb->s_bdi, BLK_RW_SYNC);
>  			clear_bdi_congested(fc->sb->s_bdi, BLK_RW_ASYNC);
>  		}
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR