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From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
To: Dave Hansen <dave.hansen@intel.com>
CC: "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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        "Zi Yan" <zi.yan@cs.rutgers.edu>, Balbir Singh <bsingharora@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Naoya Horiguchi" <nao.horiguchi@gmail.com>
Subject: Re: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7
 to bit 6
Thread-Topic: [PATCH v2 01/12] mm: x86: move _PAGE_SWP_SOFT_DIRTY from bit 7
 to bit 6
Thread-Index: AQHSOU8ruh1UI3IyjUmElP3izhZM26DSSyyAgAAblQA=
Date: Fri, 11 Nov 2016 01:08:36 +0000
Message-ID: <20161111010834.GA28679@hori1.linux.bs1.fc.nec.co.jp>
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On Thu, Nov 10, 2016 at 03:29:51PM -0800, Dave Hansen wrote:
> 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 file mapping is obsolete, so let's move _PAGE_SWP_SOFT_DIRTY to that bit.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.

Thank you for the information. According to 00839ee3b29, some bits which
are safe from the errata are reclaimed, so assigning one of such bits for
_PAGE_SWP_SOFT_DIRTY seems to work. And I'll update the description.

Thanks,
Naoya Horiguchi