Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[Minchan Kim]

On Mon, Mar 15, 2010 at 11:21 PM, Mel Gorman <mel@xxxxxxxxx> wrote:
> On Mon, Mar 15, 2010 at 09:48:49PM +0900, Minchan Kim wrote:
>> On Mon, 2010-03-15 at 11:28 +0000, Mel Gorman wrote:
>> > The use after free looks like
>> >
>> > 1. page_mapcount(page) was zero so anon_vma was no longer reliable
>> > 2. rcu lock taken but the anon_vma at this point can already be garbage because the
>> > Â Âprocess exited
>> > 3. call try_to_unmap, looks up tha anon_vma and locks it. This causes problems
>> >
>> > I thought the race would be closed but there is still a very tiny window there all
>> > right. The following alternative should close it. What do you think?
>> >
>> > Â Â Â Â if (PageAnon(page)) {
>> > Â Â Â Â Â Â rcu_read_lock();
>> >
>> > Â Â Â Â Â Â Â Â /*
>> > Â Â Â Â Â Â Â Â Â* If the page has no mappings any more, just bail. An
>> > Â Â Â Â Â Â Â Â Â* unmapped anon page is likely to be freed soon but worse,
>> > Â Â Â Â Â Â Â Â Â* it's possible its anon_vma disappeared between when
>> > Â Â Â Â Â Â Â Â Â* the page was isolated and when we reached here while
>> > Â Â Â Â Â Â Â Â Â* the RCU lock was not held
>> > Â Â Â Â Â Â Â Â Â*/
>> > Â Â Â Â Â Â Â Â if (!page_mapcount(page)) {
>> > Â Â Â Â Â Â Â Â Â Â rcu_read_unlock();
>> > Â Â Â Â Â Â Â Â Â Â Â Â goto uncharge;
>> > Â Â Â Â Â Â }
>> >
>> > Â Â Â Â Â Â Â Â rcu_locked = 1;
>> > Â Â Â Â Â Â Â Â anon_vma = page_anon_vma(page);
>> > Â Â Â Â Â Â Â Â atomic_inc(&anon_vma->external_refcount);
>> > Â Â Â Â }
>> >
>> > The rcu_unlock label is not used here because the reference counts were not taken in
>> > the case where page_mapcount == 0.
>> >
>>
>> Please, repost above code with your use-after-free scenario comment.
>>
>
> This will be the replacement patch so.
>
> ==== CUT HERE ====
> mm,migration: Do not try to migrate unmapped anonymous pages
>
> rmap_walk_anon() was triggering errors in memory compaction that look like
> use-after-free errors. The problem is that between the page being isolated
> from the LRU and rcu_read_lock() being taken, the mapcount of the page
> dropped to 0 and the anon_vma gets freed. This can happen during memory
> compaction if pages being migrated belong to a process that exits before
> migration completes. Hence, the use-after-free race looks like
>
> Â1. Page isolated for migration
> Â2. Process exits
> Â3. page_mapcount(page) drops to zero so anon_vma was no longer reliable
> Â4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage
> Â4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock
> Â Âis garbage.
>
> This patch checks the mapcount after the rcu lock is taken. If the
> mapcount is zero, the anon_vma is assumed to be freed and no further
> action is taken.
>
> Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>


-- 
Kind regards,
Minchan Kim
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