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.
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
Thx,
-Kame
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