dm: improve documentation and code clarity in dm_merge_bvec (148e51ba) · Commits · 戴 / test

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:

Mike Snitzer <snitzer@redhat.com>

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;

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