xfs: remove a superflous hash lookup when inserting new buffers

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@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 374c4e508b12767047917039fd2749ddfd6213af..1a6542e01becaac5f29415b0bea68aed6b5e8632 100644 (file)
--- 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;