md/raid10: improve discard request for far layout

For far layout, the discard region is not continuous on disks. So it needs
far copies r10bio to cover all regions. It needs a way to know all r10bios
have finish or not. Similar with raid10_sync_request, only the first r10bio
master_bio records the discard bio. Other r10bios master_bio record the
first r10bio. The first r10bio can finish after other r10bios finish and
then return the discard bio.

Tested-by: Adrian Huang <[e-mail]>
Signed-off-by: Xiao Ni <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>

drivers/md/raid10.c

···
 		raid10_write_request(mddev, bio, r10_bio);
 }
 
+static void raid_end_discard_bio(struct r10bio *r10bio)
+{
+	struct r10conf *conf = r10bio->mddev->private;
+	struct r10bio *first_r10bio;
+
+	while (atomic_dec_and_test(&r10bio->remaining)) {
+
+		allow_barrier(conf);
+
+		if (!test_bit(R10BIO_Discard, &r10bio->state)) {
+			first_r10bio = (struct r10bio *)r10bio->master_bio;
+			free_r10bio(r10bio);
+			r10bio = first_r10bio;
+		} else {
+			md_write_end(r10bio->mddev);
+			bio_endio(r10bio->master_bio);
+			free_r10bio(r10bio);
+			break;
+		}
+	}
+}
+
 static void raid10_end_discard_request(struct bio *bio)
 {
 	struct r10bio *r10_bio = bio->bi_private;
···
 		rdev = conf->mirrors[dev].rdev;
 	}
 
-	if (atomic_dec_and_test(&r10_bio->remaining)) {
-		md_write_end(r10_bio->mddev);
-		raid_end_bio_io(r10_bio);
-	}
-
+	raid_end_discard_bio(r10_bio);
 	rdev_dec_pending(rdev, conf->mddev);
 }
 
···
 {
 	struct r10conf *conf = mddev->private;
 	struct geom *geo = &conf->geo;
-	struct r10bio *r10_bio;
+	int far_copies = geo->far_copies;
+	bool first_copy = true;
+	struct r10bio *r10_bio, *first_r10bio;
 	struct bio *split;
 	int disk;
 	sector_t chunk;
···
 		wait_barrier(conf);
 	}
 
-	r10_bio = mempool_alloc(&conf->r10bio_pool, GFP_NOIO);
-	r10_bio->mddev = mddev;
-	r10_bio->state = 0;
-	r10_bio->sectors = 0;
-	memset(r10_bio->devs, 0, sizeof(r10_bio->devs[0]) * geo->raid_disks);
-
-	wait_blocked_dev(mddev, r10_bio);
-
-	r10_bio->master_bio = bio;
-
 	bio_start = bio->bi_iter.bi_sector;
 	bio_end = bio_end_sector(bio);
 
···
 		last_stripe_index *= geo->far_copies;
 	end_disk_offset = (bio_end & geo->chunk_mask) +
 				(last_stripe_index << geo->chunk_shift);
+
+retry_discard:
+	r10_bio = mempool_alloc(&conf->r10bio_pool, GFP_NOIO);
+	r10_bio->mddev = mddev;
+	r10_bio->state = 0;
+	r10_bio->sectors = 0;
+	memset(r10_bio->devs, 0, sizeof(r10_bio->devs[0]) * geo->raid_disks);
+	wait_blocked_dev(mddev, r10_bio);
+
+	/*
+	 * For far layout it needs more than one r10bio to cover all regions.
+	 * Inspired by raid10_sync_request, we can use the first r10bio->master_bio
+	 * to record the discard bio. Other r10bio->master_bio record the first
+	 * r10bio. The first r10bio only release after all other r10bios finish.
+	 * The discard bio returns only first r10bio finishes
+	 */
+	if (first_copy) {
+		r10_bio->master_bio = bio;
+		set_bit(R10BIO_Discard, &r10_bio->state);
+		first_copy = false;
+		first_r10bio = r10_bio;
+	} else
+		r10_bio->master_bio = (struct bio *)first_r10bio;
 
 	rcu_read_lock();
 	for (disk = 0; disk < geo->raid_disks; disk++) {
···
 		}
 	}
 
-	if (atomic_dec_and_test(&r10_bio->remaining)) {
-		md_write_end(r10_bio->mddev);
-		raid_end_bio_io(r10_bio);
+	if (!geo->far_offset && --far_copies) {
+		first_stripe_index += geo->stride >> geo->chunk_shift;
+		start_disk_offset += geo->stride;
+		last_stripe_index += geo->stride >> geo->chunk_shift;
+		end_disk_offset += geo->stride;
+		atomic_inc(&first_r10bio->remaining);
+		raid_end_discard_bio(r10_bio);
+		wait_barrier(conf);
+		goto retry_discard;
 	}
+
+	raid_end_discard_bio(r10_bio);
 
 	return 0;
 out:

drivers/md/raid10.h

···
 	R10BIO_Previous,
 /* failfast devices did receive failfast requests. */
 	R10BIO_FailFast,
+	R10BIO_Discard,
 };
 #endif