linux-kernel - Re: [RFC] rmap: fix "race" between do_wp_page and shrink_active

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Date:	Sun, 17 May 2015 20:44:12 +0800
From:	yalin <yalin.wang2010@...il.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
CC:	Rik van Riel <riel@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] rmap: fix "race" between do_wp_page and shrink_active_list


> On Mon, May 11, 2015 at 04:59:27PM +0800, yalin wang wrote:
>> i am confused about your analysis ,
>> for the race stack:
>>
>> CPU0                          CPU1
>>
>>     ----                          ----
>>
>>     do_wp_page                    shrink_active_list
>>
>>      lock_page                     page_referenced
>>
>>                                     PageAnon->yes, so skip trylock_page
>>
>>      page_move_anon_rmap
>>
>>       page->mapping = anon_vma
>>
>>                                     rmap_walk
>>
>>                                      PageAnon->no
>>
>>                                      rmap_walk_file
>>
>>                                       BUG
>>
>>       page->mapping += PAGE_MAPPING_ANON
>>
>> the page should must change from PageAnon() to !PageAnon() when crash happened.
>> but page_move_anon_rmap() is doing change a page from !PageAnon()
>> (swapcache page) to PageAnon() ,
> A swapcache page is not necessarily !PageAnon. In do_wp_page() old_page
> *is* PageAnon. It may or may not be on the swapcache though, which does
> not really matter.
>
>> how does this race condition crash happened ?
> It never happened. It might theoretically happen due to a compiler
> "optimization" I described above.
i see,
Thanks for your explanation!

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