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From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
To: Michal Hocko <mhocko@kernel.org>
CC: "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
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        Mel Gorman <mgorman@techsingularity.net>,
        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" <linux-kernel@vger.kernel.org>,
        Naoya Horiguchi <nao.horiguchi@gmail.com>
Subject: Re: [PATCH v2 04/12] mm: thp: introduce
 CONFIG_ARCH_ENABLE_THP_MIGRATION
Thread-Topic: [PATCH v2 04/12] mm: thp: introduce
 CONFIG_ARCH_ENABLE_THP_MIGRATION
Thread-Index: AQHSOU807s/AHkLIAEyyqET6YZn3OqDt/AoAgAEk3gA=
Date: Tue, 29 Nov 2016 07:50:06 +0000
Message-ID: <20161129075006.GA15582@hori1.linux.bs1.fc.nec.co.jp>
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On Mon, Nov 28, 2016 at 03:21:54PM +0100, Michal Hocko wrote:
> On Tue 08-11-16 08:31:49, Naoya Horiguchi wrote:
> > Introduces CONFIG_ARCH_ENABLE_THP_MIGRATION to limit thp migration
> > functionality to x86_64, which should be safer at the first step.
> 
> Please make sure to describe why this has to be arch specific and what
> are arches supposed to provide in order to enable this option.

OK, the below will be added in the future version:

  Thp migration is an arch-specific feature because it depends on the
  arch-dependent behavior of non-present format of page table entry.
  What you need to enable this option in other archs are:
  - to define arch-specific transformation functions like __pmd_to_swp_entry()
    and __swp_entry_to_pmd(),
  - to make sure that arch-specific page table walking code can properly handle
    !pmd_present case (gup_pmd_range() is a good example),
  - (if your archs enables CONFIG_HAVE_ARCH_SOFT_DIRTY,) to define soft dirty
    routines like pmd_swp_mksoft_dirty.

Thanks,
Naoya Horiguchi