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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > > > Kosaki-san,
> > > > 
> > > >  IIUC, the race in memory-hotunplug was fixed by this patch [2/11].
> > > > 
> > > >  But, this behavior of unmap_and_move() requires access to _freed_
> > > >  objects (spinlock). Even if it's safe because of SLAB_DESTROY_BY_RCU,
> > > >  it't not good habit in general.
> > > > 
> > > >  After direct compaction, page-migration will be one of "core" code of
> > > >  memory management. Then, I agree to patch [1/11] as our direction for
> > > >  keeping sanity and showing direction to more updates. Maybe adding
> > > >  refcnt and removing RCU in futuer is good.
> > > 
> > > But Christoph seems oppose to remove SLAB_DESTROY_BY_RCU. then refcount
> > > is meaningless now.
> > 
> > Christoph is opposed to removing it because of cache-hotness issues more
> > so than use-after-free concerns. The refcount is needed with or without
> > SLAB_DESTROY_BY_RCU.
> > 
> 
> I wonder a code which the easiest to be read will be like following.
> ==
> 
>         if (PageAnon(page)) {
>                 struct anon_vma anon = page_lock_anon_vma(page);
> 		/* to take this lock, this page must be mapped. */
> 		if (!anon_vma)
> 			goto uncharge;
> 		increase refcnt
> 		page_unlock_anon_vma(anon);
>         }
> 	....
> ==

This seems very good and acceptable to me. This refcnt usage
obviously reduce rcu-lock holding time.

I still think no refcount doesn't cause any disaster. but I agree
this is forward step patch.

thanks.


> and
> ==
> void anon_vma_free(struct anon_vma *anon)
> {
> 	/*
> 	 * To increase refcnt of anon-vma, anon_vma->lock should be held by
> 	 * page_lock_anon_vma(). It means anon_vma has a "mapped" page.
> 	 * If this anon is freed by unmap or exit, all pages under this anon
> 	 * must be unmapped. Then, just checking refcnt without lock is ok.
> 	 */
> 	if (check refcnt > 0)
> 		return do nothing
> 	kmem_cache_free(anon);
> }
> ==
> 
> Then, rcu_read_lock can be removed in clean way.
> 
> Thanks,
> -Kame
> 
> 
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