[PATCH] cfq-iosched: tighten allow merge criteria

The logic in cfq_allow_merge() wasn't clear enough - basically allow
merging for the same queues only.  Do a fast check for 'rq and bio both
sync/async' before doing the cfqq hash lookup.

This is verified to work with the fixed elv_try_merge() from commit
bb4067e34159648d394943d5e2a011f838bff22f.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 13 deletions

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 9fc5eafa6c0e..4b4217d9be78 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -577,25 +577,20 @@ static int cfq_allow_merge(request_queue_t *q, struct request *rq,
         pid_t key;
 
         /*
-         * If bio is async or a write, always allow merge
+         * Disallow merge, if bio and rq aren't both sync or async
          */
-        if (!bio_sync(bio) || rw == WRITE)
-                return 1;
-
-        /*
-         * bio is sync. if request is not, disallow.
-         */
-        if (!rq_is_sync(rq))
+        if (!!bio_sync(bio) != !!rq_is_sync(rq))
                 return 0;
 
         /*
-         * Ok, both bio and request are sync. Allow merge if they are
-         * from the same queue.
+         * Lookup the cfqq that this bio will be queued with. Allow
+         * merge only if rq is queued there.
          */
-        key = cfq_queue_pid(current, rw, 1);
+        key = cfq_queue_pid(current, rw, bio_sync(bio));
         cfqq = cfq_find_cfq_hash(cfqd, key, current->ioprio);
-        if (cfqq != RQ_CFQQ(rq))
-                return 0;
+
+        if (cfqq == RQ_CFQQ(rq))
+                return 1;
 
         return 1;
 }