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
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1607,9 +1607,9 @@
* 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 @@
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;