Re: [PATCH v4 08/11] block: kill merge_bvec_fn() completely

On Fri, May 22, 2015 at 11:18:40AM -0700, Ming Lin wrote:

From: Kent Overstreet <redacted>

As generic_make_request() is now able to handle arbitrarily sized bios,
it's no longer necessary for each individual block driver to define its
own ->merge_bvec_fn() callback. Remove every invocation completely.

It might be good to replace patch 1 and this one by a patch per driver
to remove the merge_bvec_fn instance and add the blk_queue_split call
for all those drivers that actually had a ->merge_bvec_fn.  As some
of them were non-trivial attention from the maintainers would be helpful,
and a patch per driver might help with that.

-/* This is called by bio_add_page().
- *
- * q->max_hw_sectors and other global limits are already enforced there.
- *
- * We need to call down to our lower level device,
- * in case it has special restrictions.
- *
- * We also may need to enforce configured max-bio-bvecs limits.
- *
- * As long as the BIO is empty we have to allow at least one bvec,
- * regardless of size and offset, so no need to ask lower levels.
- */
-int drbd_merge_bvec(struct request_queue *q, struct bvec_merge_data *bvm, struct bio_vec *bvec)

This just checks the lower device, so it looks obviously fine.

-static int pkt_merge_bvec(struct request_queue *q, struct bvec_merge_data *bmd,
-			  struct bio_vec *bvec)
-{
-	struct pktcdvd_device *pd = q->queuedata;
-	sector_t zone = get_zone(bmd->bi_sector, pd);
-	int used = ((bmd->bi_sector - zone) << 9) + bmd->bi_size;
-	int remaining = (pd->settings.size << 9) - used;
-	int remaining2;
-
-	/*
-	 * A bio <= PAGE_SIZE must be allowed. If it crosses a packet
-	 * boundary, pkt_make_request() will split the bio.
-	 */
-	remaining2 = PAGE_SIZE - bmd->bi_size;
-	remaining = max(remaining, remaining2);
-
-	BUG_ON(remaining < 0);
-	return remaining;
-}

As mentioned in the comment pkt_make_request will split the bio so pkt
looks fine.

It seems rbd handles requests spanning objects just fine, so I don't
really understand why rbd_merge_bvec even exists.  Getting some form
of ACK from the ceph folks would be useful.

-/*
- * We assume I/O is going to the origin (which is the volume
- * more likely to have restrictions e.g. by being striped).
- * (Looking up the exact location of the data would be expensive
- * and could always be out of date by the time the bio is submitted.)
- */
-static int cache_bvec_merge(struct dm_target *ti,
-			    struct bvec_merge_data *bvm,
-			    struct bio_vec *biovec, int max_size)
-{

DM seems to have the most complex merge functions of all drivers, so
I'd really love to see an ACK from Mike.