[PATCH 6.8 038/715] md/raid1: fix choose next idle in read_balance()

Sasha Levin
2024-03-24T22:35:37Z

From: Yu Kuai <yukuai3@huawei.com>

[ Upstream commit 257ac239ffcfd097a9a0732bf5095fb00164f334 ]

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 || 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 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) 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: 2e52d449bcec ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/raid1.c | 32 ++++++++++++++++++++++----------
1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 456d745eb93b3..82c9bb404ccca 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -601,13 +601,12 @@ 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
@@ -618,11 +617,11 @@ 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) ||
@@ -715,7 +714,6 @@ 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
@@ -734,15 +732,22 @@ 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;
}
- break;
}

- if (choose_next_idle)
- continue;
-
if (min_pending > pending) {
min_pending = pending;
best_pending_disk = disk;
@@ -754,6 +759,13 @@ 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
--
2.43.0