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: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 58f3927..0fee0e5 100644 (file)
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -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;