On Fri, Nov 12, 2021 at 09:21:40AM +0100, Christoph Hellwig wrote:

On Fri, Nov 12, 2021 at 04:11:37PM +0800, Ming Lei wrote:

quoted

@@ -2564,13 +2564,15 @@ static inline struct request *blk_mq_get_request(struct request_queue *q,
 			if (blk_mq_attempt_bio_merge(q, bio, nsegs,
 						same_queue_rq))
 				return NULL;
+			if (bio->bi_opf != rq->cmd_flags)
+				goto fallback;

I think this deserves a comment, as this means a read prealloc

OK.

can only be used for reads, and no fua can be set if the preallocating
I/O didn't use fua, etc.

What are the pitfalls of just chanigng cmd_flags?

Then we need to check cmd_flags carefully, such as hctx->type has to
be same, flush & passthrough flags has to be same, that said all
->cmd_flags used for allocating rqs have to be same with the following
bio->bi_opf.

In usual cases, I guess all IOs submitted from same plug batch should be
same type. If not, we can switch to change cmd_flags.


Thanks,
Ming