On Mon, Feb 26 2018 at 5:56pm -0500, Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > The block layer provides a function direct_make_request - but it doesn't > provide any test whether this function may be used or not. > > Device mapper currently uses a dirty trick - it compares the result of > bdevname against the string "nvme" to test if direct_make_request can be > used. dm's DM_TYPE_NVME_BIO_BASED is backed by that crude "nvme" check _but_ it is a means to an end. Alternative would be to have NVMe (and other devices) set something like QUEUE_FLAG_NO_PARTIAL_COMPLETION or something -- and then DM would need to verify all underlying devices set that flag. > The information whether the driver will or won't recurse can be easily > obtained in generic_make_request (by comparing make_request_fn - or > alternatively, we could introduce a queue flag for this), so my suggestion > is to just delete direct_make_request and fold this "norecurse" > optimization directly into generic_make_request This will allow us to > simplify device mapper paths and get rid of device name matching. Sorry, Nack from me as implemented. Please see below. > Index: linux-2.6/block/blk-core.c > =================================================================== > --- linux-2.6.orig/block/blk-core.c 2018-02-26 20:37:19.088499000 +0100 > +++ linux-2.6/block/blk-core.c 2018-02-26 20:43:57.839999000 +0100 > @@ -2265,6 +2265,8 @@ end_io: > return false; > } > > +blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio); > + > /** > * generic_make_request - hand a buffer to its device driver for I/O > * @bio: The bio describing the location in memory and on the device. > @@ -2300,10 +2302,14 @@ blk_qc_t generic_make_request(struct bio > */ > struct bio_list bio_list_on_stack[2]; > blk_qc_t ret = BLK_QC_T_NONE; > + bool may_recurse; > > if (!generic_make_request_checks(bio)) > goto out; > > + may_recurse = bio->bi_disk->queue->make_request_fn != blk_queue_bio && > + bio->bi_disk->queue->make_request_fn != blk_mq_make_request; > + This does _not_ allow dm's DM_TYPE_NVME_BIO_BASED to make use of your direct_make_request() equivalent that you've encoded into generic_make_request(). There is no easy way to _know_, at runtime, that an arbitrary queue (e.g. stacked DM device) does _not_ split IO (therefore not needing to recurse). Same can be said of NVMe given NVMe multipath sets its make_request_fn to nvme_ns_head_make_request(). So for both cases that don't need to recurse you're setting @may_recurse to true. As such, this patch is not equivalent to what we have now. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
