[PATCH v4 8/9] blk-merge: Ensure bios aren't split in middle of a crypto data unit

[Satya Tangirala <satyaprateek2357@gmail.com>]

From: Satya Tangirala <satyat@google.com>

Update get_max_io_size() to return a value aligned to
bio_required_sector_alignment(). With this change, and the changes
to blk_ksm_register() that restrict the supported data unit sizes
based on the queue's limits, blk_bio_segment_split() won't split bios in
the middle of a data unit anymore

Signed-off-by: Satya Tangirala <satyat@google.com>
---
 block/blk-merge.c | 49 ++++++++++++++++++++++++++++++-----------------
 1 file changed, 31 insertions(+), 18 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index a11b3b53717e..68c96dec5680 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -135,27 +135,39 @@ static struct bio *blk_bio_write_same_split(struct request_queue *q,
 
 /*
  * Return the maximum number of sectors from the start of a bio that may be
- * submitted as a single request to a block device. If enough sectors remain,
- * align the end to the physical block size. Otherwise align the end to the
- * logical block size. This approach minimizes the number of non-aligned
- * requests that are submitted to a block device if the start of a bio is not
- * aligned to a physical block boundary.
+ * submitted as a single request to a block device. Tries to align the end to
+ * the physical block size, while also aligning the returned number of sectors
+ * to bio_required_sector_alignment(). This approach minimizes the number of
+ * non-aligned requests that are submitted to a block device if the start of a
+ * bio is not aligned to a physical block boundary.
+ *
+ * More clearly, there are two conditions we're interested in satisfying.
+ *
+ * Condition 1) We absolutely must have @return divisible by the
+ * bio_required_sector_alignment(bio).
+ *
+ * Condition 2) *If possible*, while still satisfying Condition 1, we would like
+ * to have start_offset + @return divisible by physical block size in sectors
+ * (pbs).
  */
 static inline unsigned get_max_io_size(struct request_queue *q,
 				       struct bio *bio)
 {
-	unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector, 0);
-	unsigned max_sectors = sectors;
-	unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
-	unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
-	unsigned start_offset = bio->bi_iter.bi_sector & (pbs - 1);
-
-	max_sectors += start_offset;
-	max_sectors &= ~(pbs - 1);
-	if (max_sectors > start_offset)
-		return max_sectors - start_offset;
-
-	return sectors & ~(lbs - 1);
+	unsigned int start_offset = bio->bi_iter.bi_sector;
+	unsigned int sectors = blk_max_size_offset(q, start_offset, 0);
+	unsigned int pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
+	unsigned int req_sector_align = bio_required_sector_alignment(bio);
+	unsigned int pbs_aligned_sector = round_down(start_offset + sectors, pbs);
+
+	/*
+	 * If we can return a pbs aligned endpoint while satisfying Condition 1,
+	 * then do so.
+	 */
+	if (pbs_aligned_sector > start_offset &&
+	    IS_ALIGNED(pbs_aligned_sector - start_offset, req_sector_align))
+		return pbs_aligned_sector - start_offset;
+
+	return round_down(sectors, req_sector_align);
 }
 
 static inline unsigned get_max_segment_size(const struct request_queue *q,
@@ -235,6 +247,7 @@ static bool bvec_split_segs(const struct request_queue *q,
  * following is guaranteed for the cloned bio:
  * - That it has at most get_max_io_size(@q, @bio) sectors.
  * - That it has at most queue_max_segments(@q) segments.
+ * - That the number of sectors is aligned to bio_required_sector_alignment(@bio).
  *
  * Except for discard requests the cloned bio will point at the bi_io_vec of
  * the original bio. It is the responsibility of the caller to ensure that the
@@ -286,7 +299,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 	 * big IO can be trival, disable iopoll when split needed.
 	 */
 	bio->bi_opf &= ~REQ_HIPRI;
-
+	sectors = round_down(sectors, bio_required_sector_alignment(bio));
 	return bio_split(bio, sectors, GFP_NOIO, bs);
 }
 
-- 
2.25.1