On Wed, Feb 02 2011 at  4:51pm -0500,
Vivek Goyal [off-list ref] wrote:

On Tue, Feb 01, 2011 at 05:46:12PM -0500, Mike Snitzer wrote:

quoted

Skip elevator initialization during request allocation if REQ_SORTED
is not set in the @rw_flags passed to the request allocator.

Set REQ_SORTED for all requests that may be put on IO scheduler.  Flush
requests are not put on IO scheduler so REQ_SORTED is not set for
them.

So we are doing all this so that elevator_private and flush data can
share the space through union and we can avoid increasing the size
of struct rq by 1 pointer (4 or 8 bytes depneding on arch)? 

Correct.

quoted

diff --git a/block/blk-core.c b/block/blk-core.c
index 72dd23b..f6fcc64 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -1197,6 +1202,7 @@ static int __make_request(struct request_queue *q, struct bio *bio)
 	const unsigned short prio = bio_prio(bio);
 	const bool sync = !!(bio->bi_rw & REQ_SYNC);
 	const bool unplug = !!(bio->bi_rw & REQ_UNPLUG);
+	const bool flush = !!(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
 	const unsigned long ff = bio->bi_rw & REQ_FAILFAST_MASK;
 	int where = ELEVATOR_INSERT_SORT;
 	int rw_flags;

@@ -1210,7 +1216,7 @@ static int __make_request(struct request_queue *q, struct bio *bio)
 
 	spin_lock_irq(q->queue_lock);
 
-	if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
+	if (flush) {
 		where = ELEVATOR_INSERT_FLUSH;
 		goto get_rq;
 	}

@@ -1293,6 +1299,14 @@ get_rq:
 		rw_flags |= REQ_SYNC;
 
 	/*
+	 * Set REQ_SORTED for all requests that may be put on IO scheduler.
+	 * The request allocator's IO scheduler initialization will be skipped
+	 * if REQ_SORTED is not set.
+	 */

Do you want to mention here that why do we want to avoid IO scheduler
initialization. Specifically mention that set_request() is avoided so
that elevator_private[*] are not initialized and that space can be
used by flush request data.

Sure, I'll post a v3 for this patch with that edit and your Acked-by.

Thanks,
Mike