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    SubjectRe: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages
    On Thu, Mar 18, 2010 at 09:48:08AM +0900, KOSAKI Motohiro wrote:
    > > > > + /*
    > > > > + * 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)) {
    > > > > + rcu_read_unlock();
    > > > > + goto uncharge;
    > > > > + }
    > > >
    > > > I haven't understand what prevent this check. Why don't we need following scenario?
    > > >
    > > > 1. Page isolated for migration
    > > > 2. Passed this if (!page_mapcount(page)) check
    > > > 3. Process exits
    > > > 4. page_mapcount(page) drops to zero so anon_vma was no longer reliable
    > > >
    > > > Traditionally, page migration logic is, it can touch garbarge of anon_vma, but
    > > > SLAB_DESTROY_BY_RCU prevent any disaster. Is this broken concept?
    > >
    > > The check is made within the RCU read lock. If the count is positive at
    > > that point but goes to zero due to a process exiting, the anon_vma will
    > > still be valid until rcu_read_unlock() is called.
    >
    > Thank you!
    >
    > then, this logic depend on SLAB_DESTROY_BY_RCU, not refcount.
    > So, I think we don't need your [1/11] patch.
    >
    > Am I missing something?
    >

    The refcount is still needed. The anon_vma might be valid, but the
    refcount is what ensures that the anon_vma is not freed and reused.

    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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