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Subject: Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle"
 transition
From:   Donald Buczek <buczek@molgen.mpg.de>
To:     Guoqing Jiang <guoqing.jiang@cloud.ionos.com>,
        Song Liu <song@kernel.org>, linux-raid@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        it+raid@molgen.mpg.de
References: <aa9567fd-38e1-7b9c-b3e1-dc2fdc055da5@molgen.mpg.de>
 <95fbd558-5e46-7a6a-43ac-bcc5ae8581db@cloud.ionos.com>
 <77244d60-1c2d-330e-71e6-4907d4dd65fc@molgen.mpg.de>
Message-ID: <7c5438c7-2324-cc50-db4d-512587cb0ec9@molgen.mpg.de>
Date:   Wed, 2 Dec 2020 18:28:13 +0100
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Dear Guoqing,

unfortunately the patch didn't fix the problem (unless I messed it up with my logging). This is what I used:

     --- a/drivers/md/md.c
     +++ b/drivers/md/md.c
     @@ -9305,6 +9305,14 @@ void md_check_recovery(struct mddev *mddev)
                             clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
                             goto unlock;
                     }
     +               if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) &&
     +                   (!test_bit(MD_RECOVERY_DONE, &mddev->recovery) ||
     +                    test_bit(MD_RECOVERY_CHECK, &mddev->recovery))) {
     +                       /* resync/recovery still happening */
     +                       pr_info("md: XXX BUGFIX applied\n");
     +                       clear_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
     +                       goto unlock;
     +               }
                     if (mddev->sync_thread) {
                             md_reap_sync_thread(mddev);
                             goto unlock;

With pausing and continuing the check four times an hour, I could trigger the problem after about 48 hours. This time, the other device (md0) has locked up on `echo idle > /sys/devices/virtual/block/md0/md/sync_action` , while the check of md1 is still ongoing:

     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 =  8.5% (666852112/7813894144) finish=1271.2min speed=93701K/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]
           [>....................]  check =  0.2% (19510348/7813894144) finish=253779.6min speed=511K/sec
           bitmap: 0/59 pages [0KB], 65536KB chunk

after 1 minute:

     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 =  8.6% (674914560/7813894144) finish=941.1min speed=126418K/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]
           [>....................]  check =  0.2% (19510348/7813894144) finish=256805.0min speed=505K/sec
           bitmap: 0/59 pages [0KB], 65536KB chunk

A data point, I didn't mention in my previous mail, is that the mdX_resync thread is not gone when the problem occurs:

     buczek@done:/scratch/local/linux (v5.10-rc6-mpi)$ ps -Af|fgrep [md
     root       134     2  0 Nov30 ?        00:00:00 [md]
     root      1321     2 27 Nov30 ?        12:57:14 [md0_raid6]
     root      1454     2 26 Nov30 ?        12:37:23 [md1_raid6]
     root      5845     2  0 16:20 ?        00:00:30 [md0_resync]
     root      5855     2 13 16:20 ?        00:14:11 [md1_resync]
     buczek    9880  9072  0 18:05 pts/0    00:00:00 grep -F [md
     
     buczek@done:/scratch/local/linux (v5.10-rc6-mpi)$ sudo cat /proc/5845/stack
     [<0>] md_bitmap_cond_end_sync+0x12d/0x170
     [<0>] raid5_sync_request+0x24b/0x390
     [<0>] md_do_sync+0xb41/0x1030
     [<0>] md_thread+0x122/0x160
     [<0>] kthread+0x118/0x130
     [<0>] ret_from_fork+0x1f/0x30

I guess, md_bitmap_cond_end_sync+0x12d is the `wait_event(bitmap->mddev->recovery_wait,atomic_read(&bitmap->mddev->recovery_active) == 0);` in md-bitmap.c.

Donald


On 01.12.20 10:29, Donald Buczek wrote:
> Am 30.11.20 um 03:06 schrieb 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?
> 
> Yes.
> 
>>
>>>
>>>      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.
> 
> Thanks. I've copied the original condition block to execute before the modified one and added some logging to it to see, if the change actually triggers. I will pause and unpause the check frequently on a busy machine to get this code executed more often.
> 
> Best
>    Donald
> 
>>
>> 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
> 

-- 
Donald Buczek
buczek@molgen.mpg.de
Tel: +49 30 8413 1433