linux-kernel - Re: [MD] Crash with 4.12+ kernel and high disk load -- bisected to 4ad23a976413: MD: use per-cpu counter for writes

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Date:   Tue, 08 Aug 2017 08:02:49 +0000
From:   David R <david@...olicited.net>
To:     NeilBrown <neilb@...e.com>
Cc:     Dominik Brodowski <linux@...inikbrodowski.net>,
        Shaohua Li <shli@...com>, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org
Subject: Re: [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@...e.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@...e.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