Subject: optimization for large size DISCARD bio by per-device bios 

This is a very early prototype, still needs more block layer code
modification to make it work.

Current upstream raid0_make_request() only handles TRIM/DISCARD bio by
chunk size, it meams for large raid0 device built by SSDs will call
million times generic_make_request() for the split bio. This patch
tries to combine small bios into large one if they are on same real
device and continuous on this real device, then send the combined large
bio to underlying device by single call to generic_make_request().

For example, use mkfs.xfs to trim a raid0 device built with 4 x 3TB
NVMeSSD, current upstream raid0_make_request() will call
generic_make_request() 5.7 million times, with this patch only 4 calls
to generic_make_request() is required.

This patch won't work in real world, because in block/blk-lib.c:
__blkdev_issue_discard() the original large bio will be split into
smaller ones by restriction of discard_granularity.

If some day SSD supports whole device sized discard_granularity, it
will be very interesting then...

The basic idea is, if a large discard bio received
by raid0_make_request(), for example it requests to discard chunk 1
to 24 on a raid0 device built by 4 SSDs. This large discard bio will
be split and written to each SSD as the following layout,
	SSD1: C1,C5,C9,C13,C17,C21
	SSD2: C2,C6,C10,C14,C18,C22
	SSD3: C3,C7,C11,C15,C19,C23
	SSD4: C4,C8,C12,C16,C20,C24
Current raid0 code will call generic_make_request() for 24 times for
each split bio. But it is possible to calculate the final layout of
each split bio, so we can combine all the bios into four per-SSD large
bio, like this,
	bio1 (on SSD1): C{1,5,9,13,17,21}
	bio2 (on SSD2): C{2,6,10,14,18,22}
	bio3 (on SSD3): C{3,7,11,15,19,23}
	bio4 (on SSD4): C{4,8,12,16,20,24}
Now we only need to call generic_make_request() for 4 times.

The code is not simple, I need more time to write text to complain how
it works. Currently you can treat it as a proof of concept.

Signed-off-by: Coly Li <colyli@suse.de>
---
 drivers/md/raid0.c | 224 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 224 insertions(+)

diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
index 258986a..73c5fac 100644
--- a/drivers/md/raid0.c
+++ b/drivers/md/raid0.c
@@ -452,6 +452,225 @@ static inline int is_io_in_chunk_boundary(struct mddev *mddev,
 	}
 }
 
+
+struct bio_record {
+	sector_t	bi_sector;
+	unsigned long	sectors;
+	struct md_rdev	*rdev;
+};
+
+static void handle_discard_request(struct mddev *mddev, struct bio *bio)
+{
+	struct bio_record *recs = NULL;
+	struct bio *split;
+	struct r0conf *conf = mddev->private;
+	sector_t sectors, sector;
+	struct strip_zone *first_zone;
+	int zone_idx;
+	sector_t zone_start, zone_end;
+	int nr_strip_zones = conf->nr_strip_zones;
+	int disks;
+	int first_rdev_idx = -1, rdev_idx;
+	struct md_rdev *first_rdev;
+	unsigned int chunk_sects = mddev->chunk_sectors;
+
+	sector = bio->bi_iter.bi_sector;
+	first_zone = find_zone(conf, &sector);
+	first_rdev = map_sector(mddev, first_zone, sector, &sector);
+
+	sectors = chunk_sects -
+		(likely(is_power_of_2(chunk_sects))
+		 ? (sector & (chunk_sects - 1))
+		 : sector_div(sector, chunk_sects));
+
+	/* if bio size is not exceed a chunk boundary,
+	 * simply handle it here.
+	 */
+	if (sectors >= bio_sectors(bio)) {
+		bio->bi_bdev = first_rdev->bdev;
+		bio->bi_iter.bi_sector = sector + first_zone->dev_start +
+					 first_rdev->data_offset;
+		if (unlikely(!blk_queue_discard(bdev_get_queue(bio->bi_bdev))))
+			/* Just ignore it */
+			bio_endio(bio);
+		else
+			generic_make_request(bio);
+		return;
+	}
+
+	/* bio is large enough to be split, allocate recs firstly */
+	disks = mddev->raid_disks;
+	recs = kcalloc(disks, sizeof(struct bio_record), GFP_NOIO);
+	if (recs == NULL) {
+		printk(KERN_ERR "md/raid0:%s: failed to allocate memory " \
+				"for bio_record", mdname(mddev));
+		bio->bi_error = -ENOMEM;
+		bio_endio(bio);
+		return;
+	}
+
+	zone_idx = first_zone - conf->strip_zone;
+	for (rdev_idx = 0; rdev_idx < first_zone->nb_dev; rdev_idx++) {
+		struct md_rdev *rdev;
+
+		rdev = conf->devlist[zone_idx * disks + rdev_idx];
+		recs[rdev_idx].rdev = rdev;
+		if (rdev == first_rdev)
+			first_rdev_idx = rdev_idx;
+	}
+
+	/* Restore due to sector_div */
+	sector = bio->bi_iter.bi_sector;
+	recs[first_rdev_idx].bi_sector = sector + first_zone->dev_start;
+	recs[first_rdev_idx].sectors = sectors;
+	BUG_ON(recs[first_rdev_idx].rdev != first_rdev);
+
+	/* recs[first_rdev_idx] is initialized with 'sectors', we need to
+	 * handle the rested sectors, which is sotred in 'sectors' too.
+	 */
+	sectors = bio_sectors(bio) - sectors;
+
+	/* bio may not be chunk size aligned, the split bio on first rdev
+	 * may not be chunk size aligned too. But the rested split bios
+	 * on rested rdevs must be chunk size aligned, and aligned to
+	 * round down chunk number.
+	 */
+	zone_end = first_zone->zone_end;
+	rdev_idx = first_rdev_idx + 1;
+	sector = likely(is_power_of_2(chunk_sects))
+		 ? sector & (~(chunk_sects - 1))
+		 : chunk_sects * (sector/chunk_sects);
+
+	while (rdev_idx < first_zone->nb_dev) {
+		recs[rdev_idx].bi_sector = sector + first_zone->dev_start;
+		if (sectors <= chunk_sects) {
+			recs[rdev_idx].sectors = sectors;
+			goto issue;
+		}
+		recs[rdev_idx].sectors = chunk_sects;
+		sectors -= chunk_sects;
+		rdev_idx++;
+	}
+
+	sector += chunk_sects;
+	zone_start = sector + first_zone->dev_start;
+	if (zone_start == zone_end) {
+		zone_idx++;
+		zone_start = conf->strip_zone[zone_idx].dev_start;
+	}
+
+	while (zone_idx < nr_strip_zones) {
+		int rdevs_in_zone = conf->strip_zone[zone_idx].nb_dev;
+		int chunks_per_rdev, rested_chunks, rested_sectors;
+		sector_t zone_sectors, grow_sectors;
+		int add_rested_sectors = 0;
+
+		zone_end = conf->strip_zone[zone_idx].zone_end;
+		zone_sectors = zone_end - zone_start;
+		chunks_per_rdev = sectors;
+		rested_sectors =
+			sector_div(chunks_per_rdev, chunk_sects * rdevs_in_zone);
+		rested_chunks = rested_sectors;
+		rested_sectors = sector_div(rested_chunks, chunk_sects);
+
+		if ((chunks_per_rdev * chunk_sects) > zone_sectors)
+			chunks_per_rdev = zone_sectors/chunk_sects;
+
+		/* rested_chunks and rested_sectors go into next zone, we won't
+		 * handle them in this zone. Set them to 0.
+		 */
+		if ((chunks_per_rdev * chunk_sects) == zone_sectors &&
+		    (rested_chunks != 0 || rested_sectors != 0)) {
+			if (rested_chunks != 0)
+				rested_chunks = 0;
+			if (rested_sectors != 0)
+				rested_sectors = 0;
+		}
+
+		if (rested_chunks == 0 && rested_sectors != 0)
+			add_rested_sectors = 1;
+
+		for (rdev_idx = 0; rdev_idx < rdevs_in_zone; rdev_idx++) {
+			/* if .sectors is not initailized (== 0), it indicates
+			 * .bi_sector is not initialized neither. We initiate
+			 * .bi_sector firstly, then set .sectors by
+			 * grow_sectors.
+			 */
+			if (recs[rdev_idx].sectors == 0)
+				recs[rdev_idx].bi_sector = zone_start;
+			grow_sectors = chunks_per_rdev * chunk_sects;
+			if (rested_chunks) {
+				grow_sectors += chunk_sects;
+				rested_chunks--;
+				if (rested_chunks == 0 &&
+				    rested_sectors != 0) {
+					recs[rdev_idx].sectors += grow_sectors;
+					sectors -= grow_sectors;
+					add_rested_sectors = 1;
+					continue;
+				}
+			}
+
+			/* if add_rested_sectors != 0, it indicates
+			 * rested_sectors != 0
+			 */
+			if (add_rested_sectors)
+				grow_sectors += rested_sectors;
+			recs[rdev_idx].sectors += grow_sectors;
+			sectors -= grow_sectors;
+			if (add_rested_sectors)
+				break;
+		}
+
+		if (sectors == 0)
+			break;
+		zone_start = zone_end;
+		zone_idx++;
+		BUG_ON(zone_start != conf->strip_zone[zone_idx].dev_start);
+	}
+
+
+issue:
+	/* recs contains the re-ordered requests layout, now we can
+	 * chain split bios from recs
+	 */
+	for (rdev_idx = 0; rdev_idx < disks; rdev_idx++) {
+		if (rdev_idx == first_rdev_idx ||
+		    recs[rdev_idx].sectors == 0)
+			continue;
+		split = bio_split(bio,
+				  recs[rdev_idx].sectors,
+				  GFP_NOIO,
+				  fs_bio_set);
+		if (split == NULL)
+			break;
+		bio_chain(split, bio);
+		BUG_ON(split->bi_iter.bi_size != recs[rdev_idx].sectors << 9);
+		split->bi_bdev = recs[rdev_idx].rdev->bdev;
+		split->bi_iter.bi_sector = recs[rdev_idx].bi_sector +
+				recs[rdev_idx].rdev->data_offset;
+
+		if (unlikely(!blk_queue_discard(
+				bdev_get_queue(split->bi_bdev))))
+			/* Just ignore it */
+			bio_endio(split);
+		else
+			generic_make_request(split);
+	}
+	BUG_ON(bio->bi_iter.bi_size != recs[first_rdev_idx].sectors << 9);
+	bio->bi_iter.bi_sector = recs[first_rdev_idx].bi_sector +
+					recs[first_rdev_idx].rdev->data_offset;
+	bio->bi_bdev = recs[first_rdev_idx].rdev->bdev;
+
+	if (unlikely(!blk_queue_discard(bdev_get_queue(bio->bi_bdev))))
+		/* Just ignore it */
+		bio_endio(bio);
+	else
+		generic_make_request(bio);
+
+	kfree(recs);
+}
+
 static void raid0_make_request(struct mddev *mddev, struct bio *bio)
 {
 	struct strip_zone *zone;
@@ -463,6 +682,11 @@ static void raid0_make_request(struct mddev *mddev, struct bio *bio)
 		return;
 	}
 
+	if (unlikely(bio_op(bio) == REQ_OP_DISCARD)) {
+		handle_discard_request(mddev, bio);
+		return;
+	}
+
 	do {
 		sector_t sector = bio->bi_iter.bi_sector;
 		unsigned chunk_sects = mddev->chunk_sectors;