On Thu, Mar 25, 2010 at 07:12:29PM +0900, KAMEZAWA Hiroyuki wrote:
> On Thu, 25 Mar 2010 18:59:25 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>
> > > > > > 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.
> >
>
> BTW, by above change and the change in patch [2/11],
> "A page turnd to be SwapCache and free unmapped but not freed"
> page will be never migrated.
>
Good point.
> Mel, could you change the check as this ??
>
> if (PageAnon(page)) {
> rcu_read_lock();
> if (!page_mapcount(page)) {
> rcu_read_unlock();
> if (!PageSwapCache(page))
> goto uncharge;
> /* unmapped swap cache can be migrated */
> } else {
> ...
> }
> .....
> } else
>
There were minor changes in how the rcu_read_lock is taken and released
based on other comments. With your suggestion, the block now looks like;
if (PageAnon(page)) {
rcu_read_lock();
rcu_locked = 1;
/*
* 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) && !PageSwapCache(page))
goto rcu_unlock;
anon_vma = page_anon_vma(page);
atomic_inc(&anon_vma->external_refcount);
}
Thanks.
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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