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SubjectRe: [MD] Crash with 4.12+ kernel and high disk load -- bisected to 4ad23a976413: MD: use per-cpu counter for writes_pending
I will apply this to my home server this evening (BST) and set off a  
check. Will have results tomorrow.

Thanks for the fix!

David


Quoting NeilBrown <neilb@suse.com>:

> On Mon, Aug 07 2017, Dominik Brodowski wrote:
>
>> Neil, Shaohua,
>>
>> following up on David R's bug message: I have observed something similar
>> on v4.12.[345] and v4.13-rc4, but not on v4.11. This is a RAID1 (on bare
>> metal partitions, /dev/sdaX and /dev/sdbY linked together). In case it
>> matters: Further upwards are cryptsetup, a DM volume group, then logical
>> volumes, and then filesystems (ext4, but also happened with xfs).
>>
>> In a tedious bisect (the bug wasn't as quickly reproducible as I would like,
>> but happened when I repeatedly created large lvs and filled them with some
>> content, while compiling kernels in parallel), I was able to track this
>> down to:
>>
>>
>> commit 4ad23a976413aa57fe5ba7a25953dc35ccca5b71
>> Author: NeilBrown <neilb@suse.com>
>> Date: Wed Mar 15 14:05:14 2017 +1100
>>
>> MD: use per-cpu counter for writes_pending
>>
>> The 'writes_pending' counter is used to determine when the
>> array is stable so that it can be marked in the superblock
>> as "Clean". Consequently it needs to be updated frequently
>> but only checked for zero occasionally. Recent changes to
>> raid5 cause the count to be updated even more often - once
>> per 4K rather than once per bio. This provided
>> justification for making the updates more efficient.
>>
>> ...
>
> Thanks for the report... and for bisecting and for re-sending...
>
> I believe I have found the problem, and have sent a patch separately.
>
> If mddev->safemode == 1 and mddev->in_sync != 0, md_check_recovery()
> causes the thread that calls it to spin.
> Prior to the patch you found, that couldn't happen. Now it can,
> so it needs to be handled more carefully.
>
> While I was examining the code, I found another bug - so that is a win!
>
> Thanks,
> NeilBrown



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