Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

[KOSAKI Motohiro]

> > > +		/*
> > > +		 * 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?


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