There are complaints that raid0 discard handling is slow. Currently we
divide discard request into chunks and dispatch to underlayer disks. The
block layer will do merge to form big requests. This causes a lot of
request split/merge and uses significant CPU time.

A simple idea is to calculate the range for each raid disk for an IO
request and send a discard request to raid disks, which will avoid the
split/merge completely. Previously Coly tried the approach, but the
implementation was too complex because of raid0 zones. This patch always
split bio in zone boundary and handle bio within one zone. It simplifies
the implementation a lot.

Cc: NeilBrown <redacted>
Cc: Coly Li <redacted>
Signed-off-by: Shaohua Li <redacted>
---
 drivers/md/raid0.c | 116 ++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 102 insertions(+), 14 deletions(-)

diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
index 84e5859..d6c0bc7 100644
--- a/drivers/md/raid0.c
+++ b/drivers/md/raid0.c

@@ -385,7 +385,7 @@ static int raid0_run(struct mddev *mddev)
 		blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors);
 		blk_queue_max_write_same_sectors(mddev->queue, mddev->chunk_sectors);
 		blk_queue_max_write_zeroes_sectors(mddev->queue, mddev->chunk_sectors);
-		blk_queue_max_discard_sectors(mddev->queue, mddev->chunk_sectors);
+		blk_queue_max_discard_sectors(mddev->queue, UINT_MAX);
 
 		blk_queue_io_min(mddev->queue, mddev->chunk_sectors << 9);
 		blk_queue_io_opt(mddev->queue,

@@ -459,6 +459,95 @@ static inline int is_io_in_chunk_boundary(struct mddev *mddev,
 	}
 }
 
+static void raid0_handle_discard(struct mddev *mddev, struct bio *bio)
+{
+	struct r0conf *conf = mddev->private;
+	struct strip_zone *zone;
+	sector_t start = bio->bi_iter.bi_sector;
+	sector_t end;
+	unsigned int stripe_size;
+	sector_t first_stripe_index, last_stripe_index;
+	sector_t start_disk_offset;
+	unsigned int start_disk_index;
+	sector_t end_disk_offset;
+	unsigned int end_disk_index;
+	unsigned int disk;
+
+	zone = find_zone(conf, &start);
+
+	if (bio_end_sector(bio) > zone->zone_end) {
+		struct bio *split = bio_split(bio,
+			zone->zone_end - bio->bi_iter.bi_sector, GFP_NOIO,
+			mddev->bio_set);
+		bio_chain(split, bio);
+		generic_make_request(bio);
+		bio = split;
+		end = zone->zone_end;
+	} else
+		end = bio_end_sector(bio);
+

+		bio_chain(discard_bio, bio);
+		if (mddev->gendisk)
+			trace_block_bio_remap(bdev_get_queue(rdev->bdev),
+				discard_bio, disk_devt(mddev->gendisk),
+				bio->bi_iter.bi_sector);
+		generic_make_request(discard_bio);
+	}
+	bio_endio(bio);
+}
+
 static void raid0_make_request(struct mddev *mddev, struct bio *bio)
 {
 	struct strip_zone *zone;

@@ -473,6 +562,11 @@ static void raid0_make_request(struct mddev *mddev, struct bio *bio)
 		return;
 	}
 
+	if (unlikely((bio_op(bio) == REQ_OP_DISCARD))) {
+		raid0_handle_discard(mddev, bio);
+		return;
+	}
+
 	bio_sector = bio->bi_iter.bi_sector;
 	sector = bio_sector;
 	chunk_sects = mddev->chunk_sectors;

@@ -498,19 +592,13 @@ static void raid0_make_request(struct mddev *mddev, struct bio *bio)
 	bio->bi_iter.bi_sector = sector + zone->dev_start +
 		tmp_dev->data_offset;
 
-	if (unlikely((bio_op(bio) == REQ_OP_DISCARD) &&
-		     !blk_queue_discard(bdev_get_queue(bio->bi_bdev)))) {
-		/* Just ignore it */
-		bio_endio(bio);
-	} else {
-		if (mddev->gendisk)
-			trace_block_bio_remap(bdev_get_queue(bio->bi_bdev),
-					      bio, disk_devt(mddev->gendisk),
-					      bio_sector);
-		mddev_check_writesame(mddev, bio);
-		mddev_check_write_zeroes(mddev, bio);
-		generic_make_request(bio);
-	}
+	if (mddev->gendisk)
+		trace_block_bio_remap(bdev_get_queue(bio->bi_bdev),
+				      bio, disk_devt(mddev->gendisk),
+				      bio_sector);
+	mddev_check_writesame(mddev, bio);
+	mddev_check_write_zeroes(mddev, bio);
+	generic_make_request(bio);
 }
 
 static void raid0_status(struct seq_file *seq, struct mddev *mddev)