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@xxxxxxxxxx>
> CC: Brian Foster <bfoster@xxxxxxxxxx>
> ---
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@xxxxxxxxxx>
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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