linux-kernel - Re: [PATCH] xfs: Correctly invert xfs

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Message-ID: <20180228190821.GA14201@wotan.suse.de>
Date:   Wed, 28 Feb 2018 19:08:21 +0000
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Vratislav Bendel <vbendel@...hat.com>
Cc:     linux-xfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Brian Foster <bfoster@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

On Wed, Feb 28, 2018 at 04:49:51PM +0100, Vratislav Bendel wrote:
> The function xfs_buftarg_isolate() used by xfs buffer schrinkers 
> to determine whether a buffer should be isolated and disposed 
> from LRU list, has inverted logic.
> 
> Excerpt from xfs_buftarg_isolate():
>         /*
>          * Decrement the b_lru_ref count unless the value is already
>          * zero. If the value is already zero, we need to reclaim the
>          * buffer, otherwise it gets another trip through the LRU.
>          */
>         if (!atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>                 spin_unlock(&bp->b_lock);
>                 return LRU_ROTATE;
>         }
> 
> However, as per documentation, atomic_add_unless() returns _zero_
> if the atomic value was originally equal to the specified *unsless* value.
> 
> Ultimately causing a xfs_buffer with ->b_lru_ref == 0, to take another 
> trip around LRU, while isolating buffers with non-zero b_lru_ref.
> 
> Signed-off-by: Vratislav Bendel <vbendel@...hat.com>
> CC: Brian Foster <bfoster@...hat.com>

Can you add a respective Fixes: tag?  Also what effects are observed by
the user when this happens on the kernel log?

 Luis