Date: Fri, 8 May 2015 12:56:04 +0300
From: Vladimir Davydov <vdavydov@parallels.com>
To: Minchan Kim <minchan@kernel.org>
CC: Andrew Morton <akpm@linux-foundation.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>,
        Jonathan Corbet <corbet@lwn.net>, <linux-api@vger.kernel.org>,
        <linux-doc@vger.kernel.org>, <linux-mm@kvack.org>,
        <cgroups@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Rik van Riel <riel@redhat.com>, Hugh Dickins <hughd@google.com>,
        Christoph Lameter <cl@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH v3 3/3] proc: add kpageidle file
Message-ID: <20150508095604.GO31732@esperanza>
References: <cover.1430217477.git.vdavydov@parallels.com>
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 <20150429043536.GB11486@blaptop>
 <20150429091248.GD1694@esperanza>
 <20150430082531.GD21771@blaptop>
 <20150430145055.GB17640@esperanza>
 <20150504031722.GA2768@blaptop>
 <20150504094938.GB4197@esperanza>
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On Mon, May 04, 2015 at 07:54:59PM +0900, Minchan Kim wrote:
> So, I guess once below compiler optimization happens in __page_set_anon_rmap,
> it could be corrupt in page_refernced.
> 
> __page_set_anon_rmap:
>         page->mapping = (struct address_space *) anon_vma;
>         page->mapping = (struct address_space *)((void *)page_mapping + PAGE_MAPPING_ANON);
> 
> Because page_referenced checks it with PageAnon which has no memory barrier.
> So if above compiler optimization happens, page_referenced can pass the anon
> page in rmap_walk_file, not ramp_walk_anon. It's my theory. :)

FWIW

If such splits were possible, we would have bugs all over the kernel
IMO. An example is do_wp_page() vs shrink_active_list(). In do_wp_page()
we can call page_move_anon_rmap(), which sets page->mapping in exactly
the same fashion as above-mentioned __page_set_anon_rmap():

	anon_vma = (void *) anon_vma + PAGE_MAPPING_ANON;
	page->mapping = (struct address_space *) anon_vma;

The page in question may be on an LRU list, because nowhere in
do_wp_page() we remove it from the list, neither do we take any LRU
related locks. The page is locked, that's true, but shrink_active_list()
calls page_referenced() on an unlocked page, so according to your logic
they can race with the latter receiving a page with page->mapping equal
to anon_vma w/o PAGE_MAPPING_ANON bit set:

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+PAGE_MAPPING_ANON

However, this does not happen.

Thanks,
Vladimir
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