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, 8 Aug 2017 09:36:14 +0200
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     NeilBrown <neilb@...e.com>
Cc:     Shaohua Li <shli@...com>, David R <david@...olicited.net>,
        linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
        tj@...nel.org
Subject: Re: [MD] Crash with 4.12+ kernel and high disk load -- bisected to
 4ad23a976413: MD: use per-cpu counter for writes_pending

On Tue, Aug 08, 2017 at 05:01:28PM +1000, NeilBrown wrote:
> 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


Nice catch. Thanks! Will give it (both patches at once) a try on the test
system immediately.

Thanks,
	Dominik

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