linux-kernel - Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT

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Date:   Thu, 10 Nov 2016 15:29:51 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Emelyanov <xemul@...allels.com>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Balbir Singh <bsingharora@...il.com>,
        linux-kernel@...r.kernel.org,
        Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7 to
 bit 6

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 file mapping is obsolete, so let's move _PAGE_SWP_SOFT_DIRTY to that bit.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.