> > > > 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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