On Mon, Jun 27, 2022 at 04:08:40PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> Currently on the slow path insert we repeat the initial hash table
> lookup before we attempt the insert, resulting in a two traversals
> of the hash table to ensure the insert is valid. The rhashtable API
> provides a method for an atomic lookup and insert operation, so we
> can avoid one of the hash table traversals by using this method.
>
> Adapted from a large patch containing this optimisation by Christoph
> Hellwig.
>
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
That's pretty neat!
Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
--D
> ---
> fs/xfs/xfs_buf.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 3bcb691c6d95..3461ef3ebc1c 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -623,8 +623,15 @@ xfs_buf_find_insert(
> }
>
> spin_lock(&pag->pag_buf_lock);
> - bp = rhashtable_lookup(&pag->pag_buf_hash, cmap, xfs_buf_hash_params);
> + bp = rhashtable_lookup_get_insert_fast(&pag->pag_buf_hash,
> + &new_bp->b_rhash_head, xfs_buf_hash_params);
> + if (IS_ERR(bp)) {
> + error = PTR_ERR(bp);
> + spin_unlock(&pag->pag_buf_lock);
> + goto out_free_buf;
> + }
> if (bp) {
> + /* found an existing buffer */
> atomic_inc(&bp->b_hold);
> spin_unlock(&pag->pag_buf_lock);
> error = xfs_buf_find_lock(bp, flags);
> @@ -635,10 +642,8 @@ xfs_buf_find_insert(
> goto out_free_buf;
> }
>
> - /* The buffer keeps the perag reference until it is freed. */
> + /* The new buffer keeps the perag reference until it is freed. */
> new_bp->b_pag = pag;
> - rhashtable_insert_fast(&pag->pag_buf_hash, &new_bp->b_rhash_head,
> - xfs_buf_hash_params);
> spin_unlock(&pag->pag_buf_lock);
> *bpp = new_bp;
> return 0;
> --
> 2.36.1
>
