Subject: Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to
 bit 6
To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, linux-mm@kvack.org
References: <1478561517-4317-1-git-send-email-n-horiguchi@ah.jp.nec.com>
 <1478561517-4317-2-git-send-email-n-horiguchi@ah.jp.nec.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Michal Hocko <mhocko@kernel.org>, Vlastimil Babka <vbabka@suse.cz>,
        Pavel Emelyanov <xemul@parallels.com>, Zi Yan <zi.yan@cs.rutgers.edu>,
        Balbir Singh <bsingharora@gmail.com>, linux-kernel@vger.kernel.org,
        Naoya Horiguchi <nao.horiguchi@gmail.com>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <534caa72-c109-9716-15d2-5e80f4038f8d@intel.com>
Date: Thu, 10 Nov 2016 15:29:51 -0800
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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 file mapping is obsolete, so let's move _PAGE_SWP_SOFT_DIRTY to that bit.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.