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

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Date:	Tue, 12 May 2015 11:34:38 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	yalin wang <yalin.wang2010@...il.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.

Thanks,
Vladimir
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