linux-kernel - Re: [RFC][PATCH] release mmap_sem before starting migration (Was Re: Need to take mmap_sem lock in move

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Message-ID: <498ADA5D.90201@virident.com>
Date:	Thu, 05 Feb 2009 17:53:57 +0530
From:	Swamy Gowda <swamy@...ident.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Brice.Goglin@...ia.fr,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC][PATCH] release mmap_sem before starting migration (Was
 Re: Need to take mmap_sem lock in move_pages.

KAMEZAWA Hiroyuki wrote:
> On Wed, 4 Feb 2009 10:39:19 -0500 (EST)
> Christoph Lameter <cl@...ux-foundation.org> wrote:
>
>> On Wed, 4 Feb 2009, KAMEZAWA Hiroyuki wrote:
>>
>> > mmap_sem can be released after page table walk ends.
>>
>> No. read lock on mmap_sem must be held since the migrate functions
>> manipulate page table entries. Concurrent large scale changes to the 
>> page
>> tables (splitting vmas, remapping etc) must not be possible.
>>
> Just for clarification:
>
> 1. changes in page table is not problem from the viewpoint of kernel.
>   (means no panic, no leak,...)
> 2. But this loses "atomic" aspect of migration and will allow unexpected
>   behaviors.
>   (means the page-mapping status after sys_move may not be what user 
> expects.)
>
>
> Thanks,
> -Kame
>
>
But I can't understand how user can see different page->mapping , since 
new page->mapping still holds the anon_vma pointer which should still 
contain the changes in the vma list( due to split vma etc). But, 
considering it as a problem how is it avoided in case of hotremove?

 

--Swamy


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