linux-kernel - Re: [PATCH 1/4] hugetlb, rmap: always use anon

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Message-ID: <4C8AE4DC.3030308@redhat.com>
Date:	Fri, 10 Sep 2010 22:09:32 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/4] hugetlb, rmap: always use anon_vma root pointer

On 09/10/2010 05:50 PM, Andi Kleen wrote:
> On Fri, 10 Sep 2010 10:19:24 -0700
> Linus Torvalds<torvalds@...ux-foundation.org>  wrote:
>
>> On Thu, Sep 9, 2010 at 9:23 PM, Naoya Horiguchi
>> <n-horiguchi@...jp.nec.com>  wrote:
>>> This patch applies Andrea's fix given by the following patch into
>>> hugepage rmapping code:
>>>
>>>   commit 288468c334e98aacbb7e2fb8bde6bc1adcd55e05
>>>   Author: Andrea Arcangeli<aarcange@...hat.com>
>>>   Date:   Mon Aug 9 17:19:09 2010 -0700
>>>
>>> This patch uses anon_vma->root and avoids unnecessary overwriting
>>> when anon_vma is already set up.
>>
>> Btw, why isn't the code in __page_set_anon_rmap() also doing this
>> cleaner version (ie a single "if (PageAnon(page)) return;" up front)?
>
> Perhaps I misunderstand the question, but __page_set_anon_rmap
> should handle Anon pages, shouldn't it?

__page_set_anon_rmap sets the page->mapping to be
a pointer to the anon_vma & PAGE_MAPPING_ANON.

PageAnon tests for page->mapping & PAGE_MAPPING_ANON,
ie. whether page->mapping is already pointing to an
anon_vma.

If it is, __page_set_anon_rmap should leave the page
mapping pointer alone.

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