md/raid1: fix choose next idle in read_balance()

Commit 12cee5a8 ("md/raid1: prevent merging too large request") add
the case choose next idle in read_balance():

read_balance:
 for_each_rdev
  if(next_seq_sect == this_sector || dist == 0)
  -> sequential reads
   best_disk = disk;
   if (...)
    choose_next_idle = 1
    continue;

 for_each_rdev
 -> iterate next rdev
  if (pending == 0)
   best_disk = disk;
   -> choose the next idle disk
   break;

  if (choose_next_idle)
   -> keep using this rdev if there are no other idle disk
   contine

However, commit 2e52d449 ("md/raid1: add failfast handling for reads.")
remove the code:

-               /* If device is idle, use it */
-               if (pending == 0) {
-                       best_disk = disk;
-                       break;
-               }

Hence choose next idle will never work now, fix this problem by
following:

1) don't set best_disk in this case, read_balance() will choose the best
   disk after iterating all the disks;
2) add 'pending' so that other idle disk will be chosen;
3) add a new local variable 'sequential_disk' to record the disk, and if
   there is no other idle disk, 'sequential_disk' will be chosen;

Fixes: 2e52d449 ("md/raid1: add failfast handling for reads.")
Co-developed-by: Paul Luse <paul.e.luse@linux.intel.com>
Signed-off-by: Paul Luse <paul.e.luse@linux.intel.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240229095714.926789-5-yukuai1@huaweicloud.com

drivers/md/raid1.c

@@ -598,13 +598,12 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 	const sector_t this_sector = r1_bio->sector;
 	int sectors;
 	int best_good_sectors;
-	int best_disk, best_dist_disk, best_pending_disk;
+	int best_disk, best_dist_disk, best_pending_disk, sequential_disk;
 	int disk;
 	sector_t best_dist;
 	unsigned int min_pending;
 	struct md_rdev *rdev;
 	int choose_first;
-	int choose_next_idle;
 
 	/*
 	 * Check if we can balance. We can balance on the whole
@@ -615,11 +614,11 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 	sectors = r1_bio->sectors;
 	best_disk = -1;
 	best_dist_disk = -1;
+	sequential_disk = -1;
 	best_dist = MaxSector;
 	best_pending_disk = -1;
 	min_pending = UINT_MAX;
 	best_good_sectors = 0;
-	choose_next_idle = 0;
 	clear_bit(R1BIO_FailFast, &r1_bio->state);
 
 	if ((conf->mddev->recovery_cp < this_sector + sectors) ||
@@ -712,7 +711,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 			int opt_iosize = bdev_io_opt(rdev->bdev) >> 9;
 			struct raid1_info *mirror = &conf->mirrors[disk];
 
-			best_disk = disk;
 			/*
 			 * If buffered sequential IO size exceeds optimal
 			 * iosize, check if there is idle disk. If yes, choose
@@ -731,14 +729,21 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 			    mirror->next_seq_sect > opt_iosize &&
 			    mirror->next_seq_sect - opt_iosize >=
 			    mirror->seq_start) {
-				choose_next_idle = 1;
-				continue;
-			}
+				/*
+				 * Add 'pending' to avoid choosing this disk if
+				 * there is other idle disk.
+				 */
+				pending++;
+				/*
+				 * If there is no other idle disk, this disk
+				 * will be chosen.
+				 */
+				sequential_disk = disk;
+			} else {
+				best_disk = disk;
 				break;
 			}
-
-		if (choose_next_idle)
-			continue;
+		}
 
 		if (min_pending > pending) {
 			min_pending = pending;
@@ -751,6 +756,13 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 		}
 	}
 
+	/*
+	 * sequential IO size exceeds optimal iosize, however, there is no other
+	 * idle disk, so choose the sequential disk.
+	 */
+	if (best_disk == -1 && min_pending != 0)
+		best_disk = sequential_disk;
+
 	/*
 	 * If all disks are rotational, choose the closest disk. If any disk is
 	 * non-rotational, choose the disk with less pending request even the