Messages in this thread | | | Date | Mon, 15 Mar 2010 14:34:20 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages |
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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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