Re: [PATCH 0/2] Fix BTT data corruptions after crash

[Dan Williams]

On Thu, Jan 28, 2016 at 10:12 AM, Toshi Kani <toshi.kani@xxxxxxx> 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.