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

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Message-ID: <20150622131827.GF7934@node.dhcp.inet.fi>
Date:	Mon, 22 Jun 2015 16:18:27 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	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 Thu, Jun 11, 2015 at 02:30:15PM +0200, Vlastimil Babka wrote:
> 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.

Okay, I'll add this.

> >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?

The only legitimate way scanner can get reference to a page is
get_page_unless_zero(), right?

All tail pages has zero ->_count until atomic_add() in
__split_huge_page_tail() -- get_page_unless_zero() will fail.
After the atomic_add() we don't care about ->_count value.
We already known how many references with should uncharge from 
head page.

For head page get_page_unless_zero() will succeed and we don't
mind. It's clear where reference should go after split: it will
stay on head page.

-- 
 Kirill A. Shutemov
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