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Commit 148e51ba authored by Mike Snitzer's avatar Mike Snitzer
Browse files

dm: improve documentation and code clarity in dm_merge_bvec 



These code changes do not introduce a functional change.

But bio_add_page() will never attempt to build up a bio larger than
queue_max_sectors().  Similarly, bio_get_nr_vecs() is also bound by
queue_max_sectors().  Therefore, there is no point in allowing
dm_merge_bvec() to answer "how many sectors can a bio have at this
offset?" with anything larger than queue_max_sectors().  Using
queue_max_sectors() rather than BIO_MAX_SECTORS serves to more
accurately convey the limits that are being imposed.

Also, use unlikely() to clarify the fact that the defensive code in
dm_merge_bvec() relative to max_size going negative shouldn't ever
happen -- if it does happen there is a bug in the block layer for
requesting larger than dm_merge_bvec()'s initial response for a given
offset.  Also, update a comment in dm_merge_bvec() relative to
max_hw_sectors_kb.  And fix empty newline whitespace.

Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
parent 604ea906

drivers/md/dm.c

+6 −6
Original line number Diff line number Diff line
@@ -1607,9 +1607,9 @@ static int dm_merge_bvec(struct request_queue *q, 
	 * Find maximum amount of I/O that won't need splitting

	 */

	max_sectors = min(max_io_len(bvm->bi_sector, ti),

			  (sector_t) BIO_MAX_SECTORS);

			  (sector_t) queue_max_sectors(q));

	max_size = (max_sectors << SECTOR_SHIFT) - bvm->bi_size;

	if (max_size < 0)

	if (unlikely(max_size < 0)) /* this shouldn't _ever_ happen */

		max_size = 0;



	/*
@@ -1621,10 +1621,10 @@ static int dm_merge_bvec(struct request_queue *q, 
		max_size = ti->type->merge(ti, bvm, biovec, max_size);

	/*

	 * If the target doesn't support merge method and some of the devices

	 * provided their merge_bvec method (we know this by looking at

	 * queue_max_hw_sectors), then we can't allow bios with multiple vector

	 * entries.  So always set max_size to 0, and the code below allows

	 * just one page.

	 * provided their merge_bvec method (we know this by looking for the

	 * max_hw_sectors that dm_set_device_limits may set), then we can't

	 * allow bios with multiple vector entries.  So always set max_size

	 * to 0, and the code below allows just one page.

	 */

	else if (queue_max_hw_sectors(q) <= PAGE_SIZE >> 9)

		max_size = 0;