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Date: Mon, 15 Mar 2010 15:28:15 +0900 From: Minchan Kim <minchan.kim@...il.com> To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> Cc: Mel Gorman <mel@....ul.ie>, Andrew Morton <akpm@...ux-foundation.org>, Andrea Arcangeli <aarcange@...hat.com>, Christoph Lameter <cl@...ux-foundation.org>, Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>, David Rientjes <rientjes@...gle.com>, KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>, Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages On Mon, Mar 15, 2010 at 2:34 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote: > On Mon, 15 Mar 2010 09:28:08 +0900 > Minchan Kim <minchan.kim@...il.com> wrote: > >> Hi, Mel. >> On Sat, Mar 13, 2010 at 1:41 AM, Mel Gorman <mel@....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@....ul.ie> >> > Acked-by: Rik van Riel <riel@...hat.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 > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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