Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle" transition

[Guoqing Jiang]

On 11/28/20 13:25, Donald Buczek wrote:
> Dear Linux mdraid people,
>
> we are using raid6 on several servers. Occasionally we had failures, 
> where a mdX_raid6 process seems to go into a busy loop and all I/O to 
> the md device blocks. We've seen this on various kernel versions.
>
> The last time this happened (in this case with Linux 5.10.0-rc4), I took 
> some data.
>
> The triggering event seems to be the md_check cron job trying to pause 
> the ongoing check operation in the morning with
>
>      echo idle > /sys/devices/virtual/block/md1/md/sync_action
>
> This doesn't complete. Here's /proc/stack of this process:
>
>      root@done:~/linux_problems/mdX_raid6_looping/2020-11-27# ps -fp 23333
>      UID        PID  PPID  C STIME TTY          TIME CMD
>      root     23333 23331  0 02:00 ?        00:00:00 /bin/bash 
> /usr/bin/mdcheck --continue --duration 06:00
>      root@done:~/linux_problems/mdX_raid6_looping/2020-11-27# cat 
> /proc/23333/stack
>      [<0>] kthread_stop+0x6e/0x150
>      [<0>] md_unregister_thread+0x3e/0x70
>      [<0>] md_reap_sync_thread+0x1f/0x1e0
>      [<0>] action_store+0x141/0x2b0
>      [<0>] md_attr_store+0x71/0xb0
>      [<0>] kernfs_fop_write+0x113/0x1a0
>      [<0>] vfs_write+0xbc/0x250
>      [<0>] ksys_write+0xa1/0xe0
>      [<0>] do_syscall_64+0x33/0x40
>      [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Note, that md0 has been paused successfully just before.

What is the personality of md0? Is it also raid6?

>      2020-11-27T02:00:01+01:00 done CROND[23333]: (root) CMD 
> (/usr/bin/mdcheck --continue --duration "06:00")
>      2020-11-27T02:00:01+01:00 done root: mdcheck continue checking 
> /dev/md0 from 10623180920
>      2020-11-27T02:00:01.382994+01:00 done kernel: [378596.606381] md: 
> data-check of RAID array md0
>      2020-11-27T02:00:01+01:00 done root: mdcheck continue checking 
> /dev/md1 from 11582849320
>      2020-11-27T02:00:01.437999+01:00 done kernel: [378596.661559] md: 
> data-check of RAID array md1
>      2020-11-27T06:00:01.842003+01:00 done kernel: [392996.625147] md: 
> md0: data-check interrupted.
>      2020-11-27T06:00:02+01:00 done root: pause checking /dev/md0 at 
> 13351127680
>      2020-11-27T06:00:02.338989+01:00 done kernel: [392997.122520] md: 
> md1: data-check interrupted.
>      [ nothing related following.... ]
>
> After that, we see md1_raid6 in a busy loop:
>
>      PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ 
> COMMAND
>      2376 root     20   0       0      0      0 R 100.0  0.0   1387:38 
> md1_raid6

Seems the reap sync thread was trying to stop md1_raid6 while md1_raid6 
was triggered again and again.

> Also, all processes doing I/O do the md device block.
>
> This is /proc/mdstat:
>
>      Personalities : [linear] [raid0] [raid1] [raid6] [raid5] [raid4] 
> [multipath]
>      md1 : active raid6 sdk[0] sdj[15] sdi[14] sdh[13] sdg[12] sdf[11] 
> sde[10] sdd[9] sdc[8] sdr[7] sdq[6] sdp[5] sdo[4] sdn[3] sdm[2] sdl[1]
>            109394518016 blocks super 1.2 level 6, 512k chunk, algorithm 
> 2 [16/16] [UUUUUUUUUUUUUUUU]
>            [==================>..]  check = 94.0% 
> (7350290348/7813894144) finish=57189.3min speed=135K/sec
>            bitmap: 0/59 pages [0KB], 65536KB chunk
>      md0 : active raid6 sds[0] sdah[15] sdag[16] sdaf[13] sdae[12] 
> sdad[11] sdac[10] sdab[9] sdaa[8] sdz[7] sdy[6] sdx[17] sdw[4] sdv[3] 
> sdu[2] sdt[1]
>            109394518016 blocks super 1.2 level 6, 512k chunk, algorithm 
> 2 [16/16] [UUUUUUUUUUUUUUUU]
>            bitmap: 0/59 pages [0KB], 65536KB chunk

So the RECOVERY_CHECK flag should be set, not sure if the simple changes
helps, but you may give it a try.

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 98bac4f..e2697d0 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -9300,7 +9300,8 @@ void md_check_recovery(struct mddev *mddev)
                        md_update_sb(mddev, 0);

                if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
-                   !test_bit(MD_RECOVERY_DONE, &mddev->recovery)) {
+                   (!test_bit(MD_RECOVERY_DONE, &mddev->recovery) ||
+                    test_bit(MD_RECOVERY_CHECK, &mddev->recovery))) {
                        /* resync/recovery still happening */
                        clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
                        goto unlock;

Thanks,
Guoqing