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

    > On Mon, Mar 15, 2010 at 2:34 PM, KAMEZAWA Hiroyuki
    > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
    > > 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
    > >
    >
    > 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?
    >
    I think this patch is not complete.
    I guess this patch in [1/11] is trigger for the race.
    ==
    +
    + /* Drop an anon_vma reference if we took one */
    + if (anon_vma && atomic_dec_and_lock(&anon_vma->migrate_refcount, &anon_vma->lock)) {
    + int empty = list_empty(&anon_vma->head);
    + spin_unlock(&anon_vma->lock);
    + if (empty)
    + anon_vma_free(anon_vma);
    + }
    ==
    If my understainding in above is correct, this "modify" freed anon_vma.
    Then, use-after-free happens. (In old implementation, there are no refcnt,
    so, there is no use-after-free ops.)


    So, what I can think of now is a patch like following is necessary.

    ==
    static inline struct anon_vma *anon_vma_alloc(void)
    {
    struct anon_vma *anon_vma;
    anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL);
    atomic_set(&anon_vma->refcnt, 1);
    }

    void anon_vma_free(struct anon_vma *anon_vma)
    {
    /*
    * This called when anon_vma is..
    * - anon_vma->vma_list becomes empty.
    * - incremetned refcnt while migration, ksm etc.. is dropped.
    * - allocated but unused.
    */
    if (atomic_dec_and_test(&anon_vma->refcnt))
    kmem_cache_free(anon_vma_cachep, anon_vma);
    }
    ==
    Then all things will go simple.
    Overhead is concern but list_empty() helps us much.

    Thanks,
    -Kame




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