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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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