Re: NFS corruption, fixed by echo 1 > /proc/sys/vm/drop_caches -- next debugging steps?

[James Hogan]

On Sun, Mar 12, 2017 at 06:43:47PM -0700, Matt Turner 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?

It smells a bit like a DMA / caching issue.

Can you provide a full kernel log. That might provide some information
about caching that might be relevant (e.g. does dcache have aliases?).

Cheers
James

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