[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>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 9fc5eaf..4b4217d 100644 (file)
--- 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;
 }