block: refactor discard bio size limiting

Move all the logic to limit the discard bio size into a common helper
so that it is better documented.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Acked-by: Coly Li <colyli@suse.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220415045258.199825-23-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-lib.c

@@ -10,14 +10,40 @@
 
 #include "blk.h"
 
+static sector_t bio_discard_limit(struct block_device *bdev, sector_t sector)
+{
+	unsigned int discard_granularity =
+		bdev_get_queue(bdev)->limits.discard_granularity;
+	sector_t granularity_aligned_sector;
+
+	if (bdev_is_partition(bdev))
+		sector += bdev->bd_start_sect;
+
+	granularity_aligned_sector =
+		round_up(sector, discard_granularity >> SECTOR_SHIFT);
+
+	/*
+	 * Make sure subsequent bios start aligned to the discard granularity if
+	 * it needs to be split.
+	 */
+	if (granularity_aligned_sector != sector)
+		return granularity_aligned_sector - sector;
+
+	/*
+	 * Align the bio size to the discard granularity to make splitting the bio
+	 * at discard granularity boundaries easier in the driver if needed.
+	 */
+	return round_down(UINT_MAX, discard_granularity) >> SECTOR_SHIFT;
+}
+
 int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 		sector_t nr_sects, gfp_t gfp_mask, int flags,
 		struct bio **biop)
 {
 	struct request_queue *q = bdev_get_queue(bdev);
 	struct bio *bio = *biop;
 	unsigned int op;
-	sector_t bs_mask, part_offset = 0;
+	sector_t bs_mask;
 
 	if (bdev_read_only(bdev))
 		return -EPERM;
@@ -48,36 +74,9 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
 	if (!nr_sects)
 		return -EINVAL;
 
-	/* In case the discard request is in a partition */
-	if (bdev_is_partition(bdev))
-		part_offset = bdev->bd_start_sect;
-
 	while (nr_sects) {
-		sector_t granularity_aligned_lba, req_sects;
-		sector_t sector_mapped = sector + part_offset;
-
-		granularity_aligned_lba = round_up(sector_mapped,
-				q->limits.discard_granularity >> SECTOR_SHIFT);
-
-		/*
-		 * Check whether the discard bio starts at a discard_granularity
-		 * aligned LBA,
-		 * - If no: set (granularity_aligned_lba - sector_mapped) to
-		 *   bi_size of the first split bio, then the second bio will
-		 *   start at a discard_granularity aligned LBA on the device.
-		 * - If yes: use bio_aligned_discard_max_sectors() as the max
-		 *   possible bi_size of the first split bio. Then when this bio
-		 *   is split in device drive, the split ones are very probably
-		 *   to be aligned to discard_granularity of the device's queue.
-		 */
-		if (granularity_aligned_lba == sector_mapped)
-			req_sects = min_t(sector_t, nr_sects,
-					  bio_aligned_discard_max_sectors(q));
-		else
-			req_sects = min_t(sector_t, nr_sects,
-					  granularity_aligned_lba - sector_mapped);
-
-		WARN_ON_ONCE((req_sects << 9) > UINT_MAX);
+		sector_t req_sects =
+			min(nr_sects, bio_discard_limit(bdev, sector));
 
 		bio = blk_next_bio(bio, bdev, 0, op, gfp_mask);
 		bio->bi_iter.bi_sector = sector;

block/blk.h

@@ -346,20 +346,6 @@ static inline unsigned int bio_allowed_max_sectors(struct request_queue *q)
 	return round_down(UINT_MAX, queue_logical_block_size(q)) >> 9;
 }
 
-/*
- * The max bio size which is aligned to q->limits.discard_granularity. This
- * is a hint to split large discard bio in generic block layer, then if device
- * driver needs to split the discard bio into smaller ones, their bi_size can
- * be very probably and easily aligned to discard_granularity of the device's
- * queue.
- */
-static inline unsigned int bio_aligned_discard_max_sectors(
-					struct request_queue *q)
-{
-	return round_down(UINT_MAX, q->limits.discard_granularity) >>
-			SECTOR_SHIFT;
-}
-
 /*
  * Internal io_context interface
  */