Re: [PATCH] xfs: Correctly invert xfs_buftarg LRU isolation logic

[Darrick J. Wong]

On Thu, Mar 01, 2018 at 02:48:00PM -0800, Darrick J. Wong wrote:
> 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@xxxxxxxxxx>
> > CC: Brian Foster <bfoster@xxxxxxxxxx>
> 
> Looks ok, will test...
> Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

This tests ok, but please address Brian and Luis' comments before I put
this in the upstream tream.

--D

> --D
> 
> > ---
> >  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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