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SubjectRe: [PATCH 0/2] Fix BTT data corruptions after crash
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On Thu, 2016-01-28 at 12:12 -0800, Dan Williams wrote:
> On Thu, Jan 28, 2016 at 10:12 AM, Toshi Kani <toshi.kani@hpe.com> wrote:
> > Data corruption issues were observed in tests which initiated a system
> > crash/reset while accessing BTT devices. This problem is reproducible.
> >
> > The BTT driver calls pmem_rw_bytes() to update data in pmem devices.
> > This interface calls __copy_user_nocache(), which uses non-temporal
> > stores so that the stores to pmem are persistent.
> >
> > __copy_user_nocache() uses non-temporal stores when a request size is
> > 8 bytes or larger (and is aligned by 8 bytes). The BTT driver updates
> > the BTT map table, which entry size is 4 bytes. Therefore, updates to
> > the map table entries remain cached, and are not written to pmem after
> > a crash. Since the BTT driver makes previous blocks free and uses them
> > for subsequent writes, the map table ends up pointing to blocks
> > allocated for other LBAs after a crash.
> >
> > Patch 1 extends __copy_user_nocache() to use non-temporal store for
> > 4 byte copy. This patch fixes the BTT data corruption issue.
> >
>
> Nice find!

:-)

> > Patch 2 changes arch_memcpy_to_pmem() to flush processor caches when
> > a request is not naturally aligned or is less than 4 bytes. This is
> > defensive change.
>
> I'm wondering if we should just document that this routine does not
> support unaligned transfers? Maybe backed by a debug mode that does
> the alignment check.

Yes, I agree. For this debug mode, do you have something in mind? Or
should we add a new CONFIG option like CONFIG_PMEM_DEBUG?

Thanks,
-Toshi

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