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

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Message-ID: <558832A4.6060609@suse.cz>
Date:	Mon, 22 Jun 2015 18:07:00 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
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 06/22/2015 03:18 PM, Kirill A. Shutemov wrote:
> 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.

Thanks.

>>> 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?

I think so.

> 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.

I guess that works, but it's rather non-obvious thus worth documenting 
somewhere.
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