From: Dave Chinner <dchinner@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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
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