linux-kernel - Re: NFS corruption, fixed by echo 1 > /proc/sys/vm/drop

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Date:   Thu, 7 Dec 2017 23:54:05 -0800
From:   Matt Turner <mattst88@...il.com>
To:     "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        linux-nfs@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Manuel Lauss <manuel.lauss@...il.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: NFS corruption, fixed by echo 1 > /proc/sys/vm/drop_caches --
 next debugging steps?

On Thu, Dec 7, 2017 at 11:00 PM, Matt Turner <mattst88@...il.com> wrote:
> On Sun, Mar 12, 2017 at 6:43 PM, Matt Turner <mattst88@...il.com> wrote:
>> On a Broadcom BCM91250a MIPS system I can reliably trigger NFS
>> corruption on the first file read.
>>
>> To demonstrate, I downloaded five identical copies of the gcc-5.4.0
>> source tarball. On the NFS server, they hash to the same value:
>>
>> server distfiles # md5sum gcc-5.4.0.tar.bz2*
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.1
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.2
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.3
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.4
>>
>> On the MIPS system (the NFS client):
>>
>> bcm91250a-le distfiles # md5sum gcc-5.4.0.tar.bz2.2
>> 35346975989954df8a8db2b034da610d  gcc-5.4.0.tar.bz2.2
>> bcm91250a-le distfiles # md5sum gcc-5.4.0.tar.bz2*
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.1
>> 35346975989954df8a8db2b034da610d  gcc-5.4.0.tar.bz2.2
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.3
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.4
>>
>> The first file read will contain some corruption, and it is persistent until...
>>
>> bcm91250a-le distfiles # echo 1 > /proc/sys/vm/drop_caches
>> bcm91250a-le distfiles # md5sum gcc-5.4.0.tar.bz2*
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.1
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.2
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.3
>> 4c626ac2a83ef30dfb9260e6f59c2b30  gcc-5.4.0.tar.bz2.4
>>
>> the caches are dropped, at which point it reads back properly.
>>
>> Note that the corruption is different across reboots, both in the size
>> of the corruption and the location. I saw 1900~ and 1400~ byte
>> sequences corrupted on separate occasions, which don't correspond to
>> the system's 16kB page size.
>>
>> I've tested kernels from v3.19 to 4.11-rc1+ (master branch from
>> today). All exhibit this behavior with differing frequencies. Earlier
>> kernels seem to reproduce the issue less often, while more recent
>> kernels reliably exhibit the problem every boot.
>>
>> How can I further debug this?
>
> I think I was wrong about the statement about kernels v3.19 to
> 4.11-rc1+. I found out I couldn't reproduce with 4.7-rc1 and then
> bisected to 4cd13c21b207e80ddb1144c576500098f2d5f882 ("softirq: Let
> ksoftirqd do its job"). Still reproduces with current tip of Linus'
> tree.
>
> Any ideas? The board's ethernet is an uncommon device supported by
> CONFIG_SB1250_MAC. Something about the ethernet driver maybe?

With the patch reverted on master (reverts cleanly), NFS corruption no
longer happens.