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Date: Thu, 11 Jun 2015 14:30:15 +0200
From: Vlastimil Babka <vbabka@suse.cz>
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Hugh Dickins <hughd@google.com>
CC: Dave Hansen <dave.hansen@intel.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>, Christoph Lameter <cl@gentwo.org>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Steve Capper <steve.capper@linaro.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>,
        Jerome Marchand <jmarchan@redhat.com>,
        Sasha Levin <sasha.levin@oracle.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCHv6 36/36] thp: update documentation
References: <1433351167-125878-1-git-send-email-kirill.shutemov@linux.intel.com> <1433351167-125878-37-git-send-email-kirill.shutemov@linux.intel.com>
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On 06/03/2015 07:06 PM, Kirill A. Shutemov wrote:
> The patch updates Documentation/vm/transhuge.txt to reflect changes in
> THP design.

One thing I'm missing is info about the deferred splitting.

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>   Documentation/vm/transhuge.txt | 124 +++++++++++++++++++++++------------------
>   1 file changed, 69 insertions(+), 55 deletions(-)
>
> diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
> index 6b31cfbe2a9a..2352b12cae93 100644
> --- a/Documentation/vm/transhuge.txt
> +++ b/Documentation/vm/transhuge.txt
> @@ -35,10 +35,10 @@ miss is going to run faster.
>
>   == Design ==
>
> -- "graceful fallback": mm components which don't have transparent
> -  hugepage knowledge fall back to breaking a transparent hugepage and
> -  working on the regular pages and their respective regular pmd/pte
> -  mappings
> +- "graceful fallback": mm components which don't have transparent hugepage
> +  knowledge fall back to breaking huge pmd mapping into table of ptes and,
> +  if nesessary, split a transparent hugepage. Therefore these components

         necessary
> +
> +split_huge_page uses migration entries to stabilize page->_count and
> +page->_mapcount.

Hm, what if there's some physical memory scanner taking page->_count 
pins? I think compaction shouldn't be an issue, but maybe some others?