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

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Message-ID: <20180301173939.GB34164@bfoster.bfoster>
Date:   Thu, 1 Mar 2018 12:39:39 -0500
From:   Brian Foster <bfoster@...hat.com>
To:     Vratislav Bendel <vbendel@...hat.com>
Cc:     linux-xfs@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.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.
> 

Nit:							     unless

> 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>
> ---

It might be worth pointing out in the commit log that currently isolated
buffers end up right back on the LRU once they are released, because
->b_lru_ref remains elevated. Therefore, this patch essentially fixes
that circuitous route by leaving them on the LRU as originally intended.
Otherwise this looks Ok to me:

Reviewed-by: Brian Foster <bfoster@...hat.com>

Thanks for sending the patch.

Brian

>  fs/xfs/xfs_buf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index d1da2ee9e6db..ac669a10c62f 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -1708,7 +1708,7 @@ xfs_buftarg_isolate(
>  	 * 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)) {
> +	if (atomic_add_unless(&bp->b_lru_ref, -1, 0)) {
>  		spin_unlock(&bp->b_lock);
>  		return LRU_ROTATE;
>  	}
> -- 
> 2.14.3
> 
> --
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