linux-kernel - Re: [PATCHv6 36/36] thp: update documentation

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Message-ID: <55797F57.8040001@suse.cz>
Date:	Thu, 11 Jun 2015 14:30:15 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
CC:	Dave Hansen <dave.hansen@...el.com>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...two.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Steve Capper <steve.capper@...aro.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Jerome Marchand <jmarchan@...hat.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv6 36/36] thp: update documentation

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@...ux.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?

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