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

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Date:	Thu, 28 Jan 2016 12:12:55 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Toshi Kani <toshi.kani@....com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...e.de>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Vishal L Verma <vishal.l.verma@...el.com>,
	micah.parrish@....com, brian.boylston@....com,
	X86 ML <x86@...nel.org>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Fix BTT data corruptions after crash

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