Patch "block: split bios to max possible length" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

block: split bios to max possible length

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
block-split-bios-to-max-possible-length.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Keith Busch <[email protected]>
Date: Tue, 12 Jan 2016 15:08:39 -0700
Subject: block: split bios to max possible length

From: Keith Busch <[email protected]>

commit e36f6204288088fda50d1c84830340ccb70f85ff upstream.

This splits bio in the middle of a vector to form the largest possible bio at the h/w's desired alignment, and guarantees the bio being split will have some data.

The criteria for splitting is changed from the max sectors to the h/w's optimal sector alignment if it is provided. For h/w that advertise their block storage's underlying chunk size, it's a big performance win to not submit commands that cross them. If sector alignment is not provided, this patch uses the max sectors as before.

This addresses the performance issue commit d380561113 attempted to fix, but was reverted due to splitting logic error.

---
 block/blk-merge.c |   19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

@@ -81,9 +81,6 @@ static struct bio *blk_bio_segment_split
 	struct bio *new = NULL;
 
 	bio_for_each_segment(bv, bio, iter) {
-		if (sectors + (bv.bv_len >> 9) > queue_max_sectors(q))
-			goto split;
-
 		/*
 		 * If the queue doesn't support SG gaps and adding this
 		 * offset would create a gap, disallow it.

@@ -91,6 +88,22 @@ static struct bio *blk_bio_segment_split
 		if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
 			goto split;
 
+		if (sectors + (bv.bv_len >> 9) >
+				blk_max_size_offset(q, bio->bi_iter.bi_sector)) {
+			/*
+			 * Consider this a new segment if we're splitting in
+			 * the middle of this vector.
+			 */
+			if (nsegs < queue_max_segments(q) &&
+			    sectors < blk_max_size_offset(q,
+						bio->bi_iter.bi_sector)) {
+				nsegs++;
+				sectors = blk_max_size_offset(q,
+						bio->bi_iter.bi_sector);
+			}
+			goto split;
+		}
+
 		if (bvprvp && blk_queue_cluster(q)) {
 			if (seg_size + bv.bv_len > queue_max_segment_size(q))
 				goto new_segment;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/block-split-bios-to-max-possible-length.patch

There turned out to be a problem with this patch on some block formats, and we need Ming's follow up to resolve, commit-id:
d0e5fbb01a67e400e82fefe4896ea40c6447ab98

Thanks, now queued up.

greg k-h