Messages in this thread | | | Subject | Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA | From | Peter Zijlstra <> | Date | Mon, 12 Apr 2010 21:30:21 +0200 |
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On Mon, 2010-04-12 at 20:40 +0200, Peter Zijlstra wrote:
Hmm, if interleaved like so
> struct anon_vma *page_lock_anon_vma(struct page *page) > { > struct anon_vma *anon_vma; > unsigned long anon_mapping;
page_remove_rmap() anon_vma_unlink() anon_vma_free()
So that the below will all observe the old page->mapping:
> rcu_read_lock(); > again: > anon_mapping = (unsigned long)rcu_dereference(page->mapping); > if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) > goto out; > anon_vma = (struct anon_vma *)(anon_mapping - PAGE_MAPPING_ANON); > > /* > * The RCU read lock ensures we can safely dereference anon_vma > * since it ensures the backing slab won't go away. It will however > * not guarantee it's the right object. > * > * First take the anon_vma->lock, this will, per anon_vma_unlink() > * avoid this anon_vma from being freed if it is a valid object. > */ > spin_lock(&anon_vma->lock); > > /* > * Secondly, we have to re-read page->mapping, so ensure it > * has not changed, rely on spin_lock() being at least a > * compiler barrier to force the re-read. > */ > if (unlikely(page_rmapping(page) != anon_vma)) { > spin_unlock(&anon_vma->lock); > goto again; > }
page_add_anon_rmap(), so that the page_mapped() test below would be positive,
> /* > * Ensure we read page->mapping before page->_mapcount, > * orders against atomic_add_negative() in page_remove_rmap(). > */ > smp_rmb(); > > /* > * Finally check that the page is still mapped, > * if not, this can't possibly be the right anon_vma. > */ > if (!page_mapped(page)) > goto unlock;
We could here return a non-valid and already freed anon_vma.
> return anon_vma; > > unlock: > spin_unlock(&anon_vma->lock); > out: > rcu_read_unlock(); > return NULL; > } > >
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