linux-kernel - Re: [PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest

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Date:	Tue, 13 Apr 2010 13:26:08 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Borislav Petkov <bp@...en8.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com
Subject: Re: [PATCH 4/4] anonvma: when setting up page->mapping, we need to 
	pick the _oldest_ anonvma

On Tue, Apr 13, 2010 at 1:23 PM, Minchan Kim <minchan.kim@...il.com> wrote:
> On Tue, Apr 13, 2010 at 10:08 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>>
>> On Tue, 13 Apr 2010, Johannes Weiner wrote:
>>>
>>> Would you mind pasting that nice description of the error case from your
>>> other email into that changelog?  I skimmed over the description but when
>>> I read this patch several hours later, I had to go back to that previous
>>> email to fully make sense of it.
>>
>> It now looks like this..
>>
>>                Linus
>> ---
>> From: Linus Torvalds <torvalds@...ux-foundation.org>
>> Date: Mon, 12 Apr 2010 12:44:29 -0700
>> Subject: [PATCH 4/4] anonvma: when setting up page->mapping, we need to pick the _oldest_ anonvma
>>
>> Otherwise we might be mapping in a page in a new mapping, but that page
>> (through the swapcache) would later be mapped into an old mapping too.
>> The page->mapping must be the case that works for everybody, not just
>> the mapping that happened to page it in first.
>>
>> Here's the scenario:
>>
>>  - page gets allocated/mapped by process A. Let's call the anon_vma we
>>   associate the page with 'A' to keep it easy to track.
>>
>>  - Process A forks, creating process B. The anon_vma in B is 'B', and has
>>   a chain that looks like 'B' -> 'A'. Everything is fine.
>>
>>  - Swapping happens. The page (with mapping pointing to 'A') gets swapped
>>   out (perhaps not to disk - it's enough to assume that it's just not
>>   mapped any more, and lives entirely in the swap-cache)
>>
>>  - Process B pages it in, which goes like this:
>>
>>        do_swap_page ->
>>          page = lookup_swap_cache(entry);
>>         ...
>>          set_pte_at(mm, address, page_table, pte);
>>          page_add_anon_rmap(page, vma, address);
>>
>>   And think about what happens here!
>>
>>   In particular, what happens is that this will now be the "first"
>>   mapping of that page, so page_add_anon_rmap() used to do
>>
>>        if (first)
>>                __page_set_anon_rmap(page, vma, address);
>>
>>   and notice what anon_vma it will use? It will use the anon_vma for
>>   process B!
>>
>>   What happens then? Trivial: process 'A' also pages it in (nothing
>>   happens, it's not the first mapping), and then process 'B' execve's
>>   or exits or unmaps, making anon_vma B go away.
>>
>>   End result: process A has a page that points to anon_vma B, but
>>   anon_vma B does not exist any more.  This can go on forever.  Forget
>>   about RCU grace periods, forget about locking, forget anything like
>>   that.  The bug is simply that page->mapping points to an anon_vma
>>   that was correct at one point, but was _not_ the one that was shared
>>   by all users of that possible mapping.
>>
>> Changing it to always use the deepest anon_vma in the anonvma chain gets
>> us to the safest model.
>>
>> This can be improved in certain cases: if we know the page is private to
>> just this particular mapping (for example, it's a new page, or it is the
>> only swapcache entry), we could pick the top (most specific) anon_vma.
>>
>> But that's a future optimization. Make it _work_ reliably first.
>>
>> Reviewed-by: Rik van Riel <riel@...hat.com>
>> Acked-by: Johannes Weiner <hannes@...xchg.org>
>> Tested-by: Borislav Petkov <bp@...en8.de> [ "What do you know, I think you fixed it!" ]
>> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Reviewed-by: Minchan Kim <minchan.kim@...il.com>

Sorry for mistake.
I was extremely excited. :)

-- 
Kind regards,
Minchan Kim
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