On Wed, Nov 2, 2016 at 11:09 AM, Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: > On Mon, Oct 31, 2016 at 08:29:01AM -0700, Christoph Hellwig wrote: >> On Sat, Oct 29, 2016 at 04:08:08PM +0800, Ming Lei wrote: >> > Avoid to access .bi_vcnt directly, because it may be not what >> > the driver expected any more after supporting multipage bvec. >> > >> > Signed-off-by: Ming Lei <tom.leiming@xxxxxxxxx> >> >> It would be really nice to have a comment in the code why it's >> even checking for multiple segments. > > Or ideally refactor the code to not care about multiple segments at all. The check on 'bio->bi_vcnt == 1' is introduced in commit de3ec86dff160(dm: don't start current request if it would've merged with the previous), which fixed one performance issue.[1] Looks the idea of the patch is to delay dispatching the rq if it would've merged with previous request and the rq is small(single bvec). I guess the motivation is to try to increase chance of merging with the delay. But why does the code check on 'bio->bi_vcnt == 1'? Once the bio is submitted, .bi_vcnt isn't changed any more and merging doesn't change it too. So should the check have been on blk_rq_bytes(rq)? Mike, please correct me if my understanding is wrong. [1] https://www.redhat.com/archives/dm-devel/2015-March/msg00014.html thanks, Ming Lei -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
