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Date: Sun, 17 May 2015 20:44:12 +0800
From: yalin <yalin.wang2010@gmail.com>
MIME-Version: 1.0
Subject: Re: [RFC] rmap: fix "race" between do_wp_page and shrink_active_list
References: <1431330677-24476-1-git-send-email-vdavydov@parallels.com> <CAFP4FLoPfisZib3SQeeW57U6NPxnpd=rNRgiv9OOsYDrFWd=6A@mail.gmail.com> <20150512083438.GB17628@esperanza>
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Cc: Rik van Riel <riel@redhat.com>, Hugh Dickins <hughd@google.com>, Christoph Lameter <cl@linux.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, Minchan Kim <minchan@kernel.org>, Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@suse.cz>, Greg Thelen <gthelen@google.com>, Michel Lespinasse <walken@google.com>, David Rientjes <rientjes@google.com>, Pavel Emelyanov <xemul@parallels.com>, Cyrill Gorcunov <gorcunov@openvz.org>, linux-mm@kvack.org, linux-kernel@vger.kernel.org


> 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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