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

[KAMEZAWA Hiroyuki]

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.
  1. we increase page_count().
  2. isolate it from LRU.
  3. call try_to_unmap() under rcu_read_lock(). Then, 
  4. replace pte with swp_entry_t made by PFN. under pte_lock.
  5. do migarate 
  6. remap new pages. under pte_lock()>
  7. 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
   page_remove_rmap().
   In this case, migration thread finds try_to_unmap doesn't unmap any pte.
   Then, at 6, remap pte will not work.
b) When the thread finds migrateion PTE(as swap entry) in zap_page_range().
   In this case, migration doesn't find migrateion PTE and remap fails.

Why rcu_read_lock() is necessary..
 - When page_mapcount() goes to 0, we shouldn't trust page->mapping is valid.
 - Possible cases are
	i) anon_vma (= page->mapping) is freed and used for other object.
 	ii) anon_vma (= page->mapping) is freed
	iii) anon_vma (= page->mapping) is freed and used as anon_vma again.

Here, anon_vma_cachep is created  by 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



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