Groups > linux.kernel > #1519442

Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 6

From	Dave Hansen <dave.hansen@intel.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 6
Date	2016-11-11 00:40 +0100
Message-ID	<sC76N-4It-3@gated-at.bofh.it> (permalink)
References	<sB1Ga-1xZ-7@gated-at.bofh.it> <sB1Ga-1xZ-15@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On 11/07/2016 03:31 PM, Naoya Horiguchi wrote:
> pmd_present() checks _PAGE_PSE along with _PAGE_PRESENT to avoid false negative
> return when it races with thp spilt (during which _PAGE_PRESENT is temporary
> cleared.) I don't think that dropping _PAGE_PSE check in pmd_present() works
> well because it can hurt optimization of tlb handling in thp split.
> In the current kernel, bit 6 is not used in non-present format because nonlinear
> file mapping is obsolete, so let's move _PAGE_SWP_SOFT_DIRTY to that bit.
> Bit 7 is used as reserved (always clear), so please don't use it for other
> purpose.
...
>  #ifdef CONFIG_MEM_SOFT_DIRTY
> -#define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE
> +#define _PAGE_SWP_SOFT_DIRTY	_PAGE_DIRTY
>  #else
>  #define _PAGE_SWP_SOFT_DIRTY	(_AT(pteval_t, 0))
>  #endif

I'm not sure this works.  Take a look at commit 00839ee3b29 and the
erratum it works around.  I _think_ this means that a system affected by
the erratum might see an erroneous _PAGE_SWP_SOFT_DIRTY/_PAGE_DIRTY get
set in swap ptes.

There are much worse things that can happen, but I don't think bits 5
(Accessed) and 6 (Dirty) are good choices since they're affected by the
erratum.

Thread

[PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to bit 6 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-11-08 00:40 +0100
  Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to  bit 6 Dave Hansen <dave.hansen@intel.com> - 2016-11-11 00:40 +0100
    Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7  to bit 6 Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-11-11 02:20 +0100
    Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7  to bit 6 "Kirill A. Shutemov" <kirill@shutemov.name> - 2016-11-11 12:20 +0100

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