[PATCH md-6.9 03/10] md/raid1: fix choose next idle in read_balance()

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Commit 12cee5a8a29e ("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 || disk == 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 2e52d449bcec ("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) set 'dist' to 0 so that if there is no other idle disk, and all disks
   are rotational, this disk will still be chosen;

Fixes: 2e52d449bcec ("md/raid1: add failfast handling for reads.")
Co-developed-by: Paul Luse <paul.e.luse@xxxxxxxxxxxxxxx>
Signed-off-by: Paul Luse <paul.e.luse@xxxxxxxxxxxxxxx>
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/md/raid1.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index c60ea58ae8c5..d0bc67e6d068 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -604,7 +604,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 	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
@@ -619,7 +618,6 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 	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 +710,6 @@ 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
@@ -731,15 +728,21 @@ 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.
+				 * Set 'dist' to 0, so that if there is no other
+				 * idle disk and all disks are rotational, this
+				 * disk will still be chosen.
+				 */
+				pending++;
+				dist = 0;
+			} else {
+				best_disk = disk;
+				break;
 			}
-			break;
 		}
 
-		if (choose_next_idle)
-			continue;
-
 		if (min_pending > pending) {
 			min_pending = pending;
 			best_pending_disk = disk;
-- 
2.39.2