Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()

KOSAKI Motohiro
2010-04-09T02:19:56Z

Hi

> > - Then checks page_mapped() without having any apparent defence
> > against page_mapped() becoming untrue one nanosecond later.
> >
> > - Checks page_mapped() inside the rcu_read_locked() section for
> > inscrutable reasons.
>
> Right, I think the page_mapped() stuff is just an early bail out.

FWIW, not only early bail out.
page_remove_rmap() don't have "page->mapping = NULL". page_remove_rmap's
comment says

/*
* It would be tidy to reset the PageAnon mapping here,
* but that might overwrite a racing page_add_anon_rmap
* which increments mapcount after us but sets mapping
* before us: so leave the reset to free_hot_cold_page,
* and remember that it's only reliable while mapped.
* Leaving it set also helps swapoff to reinstate ptes
* faster for those pages still in swapcache.
*/

So, If following scenario happen, we can't dereference page->mapping
(iow, can't call spin_lock(&anon_vma->lock)). It might be pointed
invalid address.

CPU0 CPU1
===============================================================
page->mappings become -1
anon_vma_unlink()
-- grace period --
page_lock_anon_vma
page_mapped()
spin_lock(&anon_vma->lock);

Of cource, this statement doesn't mean I'm against your patch at all.
I like it.