Date: Tue, 2 Jun 2015 13:51:35 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Abelardo Ricart III <aricart@memnix.com>
cc: Brandon Smith <freedom@reardencode.com>, Mike Snitzer <snitzer@redhat.com>,
        dm-devel@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: Regression: Disk corruption with dm-crypt and kernels >= 4.0
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On Mon, 18 May 2015, Abelardo Ricart III wrote:

> On Fri, 2015-05-15 at 08:04 -0700, Brandon Smith wrote:
> > On 2015-05-01 (Fri) at 19:42:15 -0400, Abelardo Ricart III wrote:
> > > > > The patchset in question was tested quite heavily so this is a
> > > > > surprising report.  I'm noticing you are opting in to dm-crypt discard
> > > > > support.  Have you tested without discards enabled?
> > > > 
> > > > I've disabled discards universally and rebuilt a vanilla kernel. After 
> > > > running
> > > > my heavy read-write-sync scripts, everything seems to be working fine now. 
> > > > I
> > > > suppose this could be something that used to fail silently before, but now
> > > > produces bad behavior? I seem to remember having something in my message 
> > > > log
> > > > about "discards not supported on this device" when running with it enabled
> > > > before.
> > > 
> > > Forgive me, but I spoke too soon. The corruption and libata errors are still
> > > there, as was evidenced when I went to reboot and got treated to an eye full 
> > > of
> > > "read-only filesystem" and ata errors.
> > > 
> > > So no, disabling discards unfortunately did nothing to help.
> > 
> > I've been experiencing the same problem.  Vanilla 4.0 series kernels,
> > dm-crypt, with/or without discards, on a ThinkPad X1 Carbon with a
> > LiteOn LGT-256M6G SSD.   
> > 
> > After some of googling around, I found some chatter relating to changes
> > in NCQ on SSDs in 4.0.   Been running w/o NCQ for a full kernel build so
> > far without issue.  Perhaps there's been some change in the interaction
> > between dm-crypt and NCQ?
> > 
> > Abelardo, can you try w/o NCQ and see if that helps your situation?
> > 
> > Best,
> > 
> > --Brandon
> 
> I've been running with NCQ disabled and been stress testing for awhile and the
> issue is indeed gone. Thanks for the workaround!
> 
> So it seems the issue is somehow related to the combination of NCQ, dm-crypt,
> and possibly (some?) SSDs.

Hi

I suspect that this is a bug in kernel NCQ processing or in SSD firmware 
and recent dm-crypt changes made the bug show up.

I suggest this:

If you have some test that reliably reproduces the bug, please do this: 
take kernel 3.19 or 3.18 and apply dm-crypt parallelization patches 
(commits f3396c58fd8442850e759843457d78b6ec3a9589, 
cf2f1abfbd0dba701f7f16ef619e4d2485de3366, 
7145c241a1bf2841952c3e297c4080b357b3e52d, 
94f5e0243c48aa01441c987743dc468e2d6eaca2, 
dc2676210c425ee8e5cb1bec5bc84d004ddf4179, 
0f5d8e6ee758f7023e4353cca75d785b2d4f6abe, 
b3c5fd3052492f1b8d060799d4f18be5a5438add) on it. If the bug doesn't show 
up with the older kernel and dm-crypt parallelization patches, use git 
bisect to find out which patch broken NCQ. When you test a kernel with 
bisect, apply the above mentioned patches to it.

Mikulas
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