Messages in this thread | | | Subject | Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma() | From | Peter Zijlstra <> | Date | Fri, 09 Apr 2010 14:57:34 +0200 |
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On Thu, 2010-04-08 at 21:17 +0200, Peter Zijlstra wrote:
> Index: linux-2.6/mm/rmap.c > =================================================================== > --- linux-2.6.orig/mm/rmap.c > +++ linux-2.6/mm/rmap.c > @@ -294,6 +294,7 @@ struct anon_vma *page_lock_anon_vma(stru > unsigned long anon_mapping; > > rcu_read_lock(); > +again: > anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > @@ -302,6 +303,12 @@ struct anon_vma *page_lock_anon_vma(stru > > anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); > spin_lock(&anon_vma->lock); > + > + if (page_rmapping(page) != anon_vma) { > + spin_unlock(&anon_vma->lock); > + goto again; > + } > + > return anon_vma; > out: > rcu_read_unlock();
OK, so I'm not quite sure about this anymore... this locking is quite,.. umh,. interesting.
We have:
- page_remove_rmap() - which decrements page->_mapcount but does not clear page->mapping. Even though both it and page_add_anon_rmap() require pte_lock, there is no guarantee these are the same locks, so these can indeed race.
So there is indeed a possibility for page->mapping to change, but holding anon_vma->lock won't make a difference.
- anon_vma->lock - this does protect vma_anon_link/unlink like things and is held during rmap walks, it only protects the list.
- SLAB_DESTROY_BY_RCU - only guarantees we can deref anon_vma, not that we'll find the one we were looking for.
- we generally unmap/zap before unlink/free - so this should not see pages race with anon_vma freeing (except when someone has a page ref), however on anon_vma merges we seem to simply unlink the next one, without updating potential pages referencing it?
- KSM seems to not do anything that swap doesn't also do (but I didn't look too closely).
- page migration seems terribly broken..
As it stands I don't see a reliable way to obtain an anon_vma, vma_address() + page_check_address() things are indeed enough, but not everybody seems to do that.
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