[PATCH 2/2] xfs: don't retry xfs_buf_find on XBF_TRYLOCK failure

[Dave Chinner <david@fromorbit.com>]

From: Dave Chinner <dchinner@redhat.com>

When looking at an event trace recently, I noticed that non-blocking
buffer lookup attempts would fail on cached locked buffers and then
run the slow cache-miss path. This means we are doing an xfs_buf
allocation, lookup and free unnecessarily every time we avoid
blocking on a locked buffer.

Fix this by changing _xfs_buf_find() to return an error status to
the caller to indicate that we failed the lock attempt rather than
just returning a NULL. This allows the higher level code to
discriminate between a cache miss and an cache hit that we failed to
lock.

This also allows us to return a -EFSCORRUPTED state if we are asked
to look up a block number outside the range of the filesystem in
_xfs_buf_find(), which moves us one step closer to being able to
handle such errors in a more graceful manner at the higher levels.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/xfs_buf.c | 93 +++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 65 insertions(+), 28 deletions(-)

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index 2ca8e2c7fbc4..41b386c26582 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -549,17 +549,31 @@ xfs_buf_hash_destroy(
 }
 
 /*
- * Look up (and insert if absent), a lockable buffer for a given
- * range of an inode.  The buffer is returned locked. No I/O is
- * implied by this call.
+ * Look up a buffer in the buffer cache and return it referenced and locked
+ * in @found_bp.
+ *
+ * If @new_bp is supplied and we have a lookup miss, insert @new_bp into the
+ * cache.
+ *
+ * If XBF_TRYLOCK is set in @flags, only try to lock the buffer and return
+ * -EAGAIN if we fail to lock it.
+ *
+ * Return values are:
+ *	-EFSCORRUPTED if have been supplied with an invalid address
+ *	-EAGAIN on trylock failure
+ *	-ENOENT if we fail to find a match and @new_bp was NULL
+ *	0, with @found_bp:
+ *		- @new_bp if we inserted it into the cache
+ *		- the buffer we found and locked.
  */
-static struct xfs_buf *
-_xfs_buf_find(
+static int
+xfs_buf_find(
 	struct xfs_buftarg	*btp,
 	struct xfs_buf_map	*map,
 	int			nmaps,
 	xfs_buf_flags_t		flags,
-	struct xfs_buf		*new_bp)
+	struct xfs_buf		*new_bp,
+	struct xfs_buf		**found_bp)
 {
 	struct xfs_perag	*pag;
 	xfs_buf_t		*bp;
@@ -567,6 +581,8 @@ _xfs_buf_find(
 	xfs_daddr_t		eofs;
 	int			i;
 
+	*found_bp = NULL;
+
 	for (i = 0; i < nmaps; i++)
 		cmap.bm_len += map[i].bm_len;
 
@@ -580,16 +596,11 @@ _xfs_buf_find(
 	 */
 	eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
 	if (cmap.bm_bn < 0 || cmap.bm_bn >= eofs) {
-		/*
-		 * XXX (dgc): we should really be returning -EFSCORRUPTED here,
-		 * but none of the higher level infrastructure supports
-		 * returning a specific error on buffer lookup failures.
-		 */
 		xfs_alert(btp->bt_mount,
 			  "%s: daddr 0x%llx out of range, EOFS 0x%llx",
 			  __func__, cmap.bm_bn, eofs);
 		WARN_ON(1);
-		return NULL;
+		return -EFSCORRUPTED;
 	}
 
 	pag = xfs_perag_get(btp->bt_mount,
@@ -604,19 +615,20 @@ _xfs_buf_find(
 	}
 
 	/* No match found */
-	if (new_bp) {
-		/* the 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);
-	} else {
+	if (!new_bp) {
 		XFS_STATS_INC(btp->bt_mount, xb_miss_locked);
 		spin_unlock(&pag->pag_buf_lock);
 		xfs_perag_put(pag);
+		return -ENOENT;
 	}
-	return new_bp;
+
+	/* the 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);
+	*found_bp = new_bp;
+	return 0;
 
 found:
 	spin_unlock(&pag->pag_buf_lock);
@@ -626,7 +638,7 @@ _xfs_buf_find(
 		if (flags & XBF_TRYLOCK) {
 			xfs_buf_rele(bp);
 			XFS_STATS_INC(btp->bt_mount, xb_busy_locked);
-			return NULL;
+			return -EAGAIN;
 		}
 		xfs_buf_lock(bp);
 		XFS_STATS_INC(btp->bt_mount, xb_get_locked_waited);
@@ -646,7 +658,8 @@ _xfs_buf_find(
 
 	trace_xfs_buf_find(bp, flags, _RET_IP_);
 	XFS_STATS_INC(btp->bt_mount, xb_get_locked);
-	return bp;
+	*found_bp = bp;
+	return 0;
 }
 
 struct xfs_buf *
@@ -656,8 +669,14 @@ xfs_incore(
 	size_t			numblks,
 	xfs_buf_flags_t		flags)
 {
+	struct xfs_buf		*bp;
+	int			error;
 	DEFINE_SINGLE_BUF_MAP(map, blkno, numblks);
-	return _xfs_buf_find(target, &map, 1, flags, NULL);
+
+	error = xfs_buf_find(target, &map, 1, flags, NULL, &bp);
+	if (error)
+		return NULL;
+	return bp;
 }
 
 /*
@@ -676,9 +695,27 @@ xfs_buf_get_map(
 	struct xfs_buf		*new_bp;
 	int			error = 0;
 
-	bp = _xfs_buf_find(target, map, nmaps, flags, NULL);
-	if (likely(bp))
+	error = xfs_buf_find(target, map, nmaps, flags, NULL, &bp);
+
+	switch (error) {
+	case 0:
+		/* cache hit */
 		goto found;
+	case -EAGAIN:
+		/* cache hit, trylock failure, caller handles failure */
+		ASSERT(flags & XBF_TRYLOCK);
+		return NULL;
+	case -ENOENT:
+		/* cache miss, go for insert */
+		break;
+	case -EFSCORRUPTED:
+	default:
+		/*
+		 * None of the higher layers understand failure types
+		 * yet, so return NULL to signal a fatal lookup error.
+		 */
+		return NULL;
+	}
 
 	new_bp = _xfs_buf_alloc(target, map, nmaps, flags);
 	if (unlikely(!new_bp))
@@ -690,8 +727,8 @@ xfs_buf_get_map(
 		return NULL;
 	}
 
-	bp = _xfs_buf_find(target, map, nmaps, flags, new_bp);
-	if (!bp) {
+	error = xfs_buf_find(target, map, nmaps, flags, new_bp, &bp);
+	if (error) {
 		xfs_buf_free(new_bp);
 		return NULL;
 	}
-- 
2.16.1