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

[Minchan Kim]

On Tue, Apr 13, 2010 at 1:23 PM, Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> On Tue, Apr 13, 2010 at 10:08 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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? ÂI 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..
>>
>> Â Â Â Â Â Â Â ÂLinus
>> ---
>> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> 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:
>>
>> Â- page gets allocated/mapped by process A. Let's call the anon_vma we
>> Â associate the page with 'A' to keep it easy to track.
>>
>> Â- Process A forks, creating process B. The anon_vma in B is 'B', and has
>> Â a chain that looks like 'B' -> 'A'. Everything is fine.
>>
>> Â- Swapping happens. The page (with mapping pointing to 'A') gets swapped
>> Â out (perhaps not to disk - it's enough to assume that it's just not
>> Â mapped any more, and lives entirely in the swap-cache)
>>
>> Â- Process B pages it in, which goes like this:
>>
>> Â Â Â Âdo_swap_page ->
>> Â Â Â Â Âpage = lookup_swap_cache(entry);
>> Â Â Â Â ...
>> Â Â Â Â Âset_pte_at(mm, address, page_table, pte);
>> Â Â Â Â Âpage_add_anon_rmap(page, vma, address);
>>
>> Â And think about what happens here!
>>
>> Â In particular, what happens is that this will now be the "first"
>> Â mapping of that page, so page_add_anon_rmap() used to do
>>
>> Â Â Â Âif (first)
>> Â Â Â Â Â Â Â Â__page_set_anon_rmap(page, vma, address);
>>
>> Â and notice what anon_vma it will use? It will use the anon_vma for
>> Â process B!
>>
>> Â What happens then? Trivial: process 'A' also pages it in (nothing
>> Â happens, it's not the first mapping), and then process 'B' execve's
>> Â or exits or unmaps, making anon_vma B go away.
>>
>> Â End result: process A has a page that points to anon_vma B, but
>> Â anon_vma B does not exist any more. ÂThis can go on forever. ÂForget
>> Â about RCU grace periods, forget about locking, forget anything like
>> Â that. ÂThe bug is simply that page->mapping points to an anon_vma
>> Â that was correct at one point, but was _not_ the one that was shared
>> Â 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@xxxxxxxxxx>
>> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
>> Tested-by: Borislav Petkov <bp@xxxxxxxxx> [ "What do you know, I think you fixed it!" ]
>> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>

Sorry for mistake.
I was extremely excited. :)

-- 
Kind regards,
Minchan Kim
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