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dm: improve documentation and code clarity in dm_merge_bvec

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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>
This commit is contained in:
Mike Snitzer 2014-10-09 19:32:22 -04:00
parent 604ea90641
commit 148e51baf8
1 changed files with 6 additions and 6 deletions
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12
drivers/md/dm.c
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@ -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;