Re: [PATCH 0/4] [RFC][v4] Workaround for Xeon Phi PTE A/D bits erratum

From: Vlastimil Babka <vbabka@suse.cz>
To: Dave Hansen <dave@sr71.net>, linux-kernel@vger.kernel.org
Cc: x86@kernel.org, linux-mm@kvack.org,
	torvalds@linux-foundation.org, akpm@linux-foundation.org,
	bp@alien8.de, ak@linux.intel.com, mhocko@suse.com,
	dave.hansen@intel.com,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [PATCH 0/4] [RFC][v4] Workaround for Xeon Phi PTE A/D bits erratum
Date: Wed, 13 Jul 2016 11:54:25 +0200	[thread overview]
Message-ID: <d4dd0222-1a7f-a5d9-60e8-d22d01a03755@suse.cz> (raw)
In-Reply-To: <20160708001909.FB2443E2@viggo.jf.intel.com>

On 07/08/2016 02:19 AM, Dave Hansen wrote:
> This patch survived a bunch of testing over the past week, including
> on hardware affected by the issue.  A debugging patch showed the
> "stray" bits being set, and no ill effects were noticed.
>
> Barring any heartburn from folks, I think this is ready for the tip
> tree.

I don't see any answer to Benjamin's question on the previous version?
https://lkml.org/lkml/2016/7/1/703

> --
>
> The Intel(R) Xeon Phi(TM) Processor x200 Family (codename: Knights
> Landing) has an erratum where a processor thread setting the Accessed
> or Dirty bits may not do so atomically against its checks for the
> Present bit.  This may cause a thread (which is about to page fault)
> to set A and/or D, even though the Present bit had already been
> atomically cleared.
>
> These bits are truly "stray".  In the case of the Dirty bit, the
> thread associated with the stray set was *not* allowed to write to
> the page.  This means that we do not have to launder the bit(s); we
> can simply ignore them.
>
> More details can be found in the "Specification Update" under "KNL4":
>
> 	http://www.intel.com/content/dam/www/public/us/en/documents/specification-updates/xeon-phi-processor-specification-update.pdf
>
> If the PTE is used for storing a swap index or a NUMA migration index,
> the A bit could be misinterpreted as part of the swap type.  The stray
> bits being set cause a software-cleared PTE to be interpreted as a
> swap entry.  In some cases (like when the swap index ends up being
> for a non-existent swapfile), the kernel detects the stray value
> and WARN()s about it, but there is no guarantee that the kernel can
> always detect it.
>
> This patch changes the kernel to attempt to ignore those stray bits
> when they get set.  We do this by making our swap PTE format
> completely ignore the A/D bits, and also by ignoring them in our
> pte_none() checks.
>
> Andi Kleen wrote the original version of this patch.  Dave Hansen
> wrote the later ones.
>
> v4: complete rework: let the bad bits stay around, but try to
>     ignore them
> v3: huge rework to keep batching working in unmap case
> v2: out of line. avoid single thread flush. cover more clear
>     cases
>
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