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SubjectRe: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages
On Mon, 15 Mar 2010 09:28:08 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> Hi, Mel.
> On Sat, Mar 13, 2010 at 1:41 AM, Mel Gorman <mel@csn.ul.ie> 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@csn.ul.ie>
> > Acked-by: Rik van Riel <riel@redhat.com>
> > ---
> >  mm/migrate.c |   10 ++++++++++
> >  1 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,
> >         * just care Anon page here.
> >         */
> >        if (PageAnon(page)) {
> > +               /*
> > +                * If the page has no mappings any more, just bail. An
> > +                * unmapped anon page is likely to be freed soon but worse,
> > +                * it's possible its anon_vma disappeared between when
> > +                * the page was isolated and when we reached here while
> > +                * the RCU lock was not held
> > +                */
> > +               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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