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

[Minchan Kim]

On Mon, Mar 15, 2010 at 2:34 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Mon, 15 Mar 2010 09:28:08 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
>
>> Hi, Mel.
>> On Sat, Mar 13, 2010 at 1:41 AM, Mel Gorman <mel@xxxxxxxxx> wrote:
>> > rmap_walk_anon() was triggering errors in memory compaction that looks like
>> > use-after-free errors in anon_vma. The problem appears to be 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 was freed. This patch
>> > skips the migration of anon pages that are not mapped by anyone.
>> >
>> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
>> > Acked-by: Rik van Riel <riel@xxxxxxxxxx>
>> > ---
>> > Âmm/migrate.c | Â 10 ++++++++++
>> > Â1 files changed, 10 insertions(+), 0 deletions(-)
>> >
>> > diff --git a/mm/migrate.c b/mm/migrate.c
>> > index 98eaaf2..3c491e3 100644
>> > --- a/mm/migrate.c
>> > +++ b/mm/migrate.c
>> > @@ -602,6 +602,16 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>> > Â Â Â Â * just care Anon page here.
>> > Â Â Â Â */
>> > Â Â Â Âif (PageAnon(page)) {
>> > + 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* unmapped anon page is likely to be freed soon but worse,
>> > + 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* the page was isolated and when we reached here while
>> > + 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 if (!page_mapcount(page))
>>
>> As looking code about mapcount of page, I got confused.
>> I think mapcount of page is protected by pte lock.
>> But I can't find pte lock in unmap_and_move.
> There is no pte_lock.
>
>> If I am right, what protects race between this condition check and
>> rcu_read_lock?
>> This patch makes race window very small but It can't remove race totally.
>>
>> I think I am missing something.
>> Pz, point me out. :)
>>
>
> Hmm. This is my understanding of old story.
>
> At migration.
> Â1. we increase page_count().
> Â2. isolate it from LRU.
> Â3. call try_to_unmap() under rcu_read_lock(). Then,
> Â4. replace pte with swp_entry_t made by PFN. under pte_lock.
> Â5. do migarate
> Â6. remap new pages. under pte_lock()>
> Â7. release rcu_read_lock().
>
> Here, we don't care whether page->mapping holds valid anon_vma or not.
>
> Assume a racy threads which calls zap_pte_range() (or some other)
>
> a) When the thread finds valid pte under pte_lock and successfully call
> Â page_remove_rmap().
> Â In this case, migration thread finds try_to_unmap doesn't unmap any pte.
> Â Then, at 6, remap pte will not work.
> b) When the thread finds migrateion PTE(as swap entry) in zap_page_range().
> Â In this case, migration doesn't find migrateion PTE and remap fails.
>
> Why rcu_read_lock() is necessary..
> Â- When page_mapcount() goes to 0, we shouldn't trust page->mapping is valid.
> Â- Possible cases are
> Â Â Â Âi) anon_vma (= page->mapping) is freed and used for other object.
> Â Â Â Âii) anon_vma (= page->mapping) is freed
> Â Â Â Âiii) anon_vma (= page->mapping) is freed and used as anon_vma again.
>
> Here, anon_vma_cachep is created Âby SLAB_DESTROY_BY_RCU. Then, possible cases
> are only ii) and iii). While anon_vma is anon_vma, try_to_unmap and remap_page
> can work well because of the list of vmas and address check. IOW, remap routine
> just do nothing if anon_vma is freed.
>
> I'm not sure by what logic "use-after-free anon_vma" is caught. But yes,
> there will be case, "anon_vma is touched after freed.", I think.
>
> Thanks,
> -Kame
>

Thanks for detail explanation, Kame.
But it can't understand me enough, Sorry.

Mel said he met "use-after-free errors in anon_vma".
So added the check in unmap_and_move.

if (PageAnon(page)) {
 ....
 if (!page_mapcount(page))
   goto uncharge;
 rcu_read_lock();

My concern what protects racy mapcount of the page?
For example,

CPU A                                 CPU B
unmap_and_move
page_mapcount check pass    zap_pte_range
<-- some stall -->                   pte_lock
<-- some stall -->                   page_remove_rmap(map_count is zero!)
<-- some stall -->                   pte_unlock
<-- some stall -->                   anon_vma_unlink
<-- some stall -->                   anon_vma free !!!!
rcu_read_lock
anon_vma has gone!!

I think above scenario make error "use-after-free", again.
What prevent above scenario?


-- 
Kind regards,
Minchan Kim
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