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Subject: Re: [PATCH 02/13] mm: Revalidate anon_vma in page_lock_anon_vma()
From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
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Date: Thu, 08 Apr 2010 23:54:26 +0200
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On Thu, 2010-04-08 at 14:20 -0700, Andrew Morton wrote:
> On Thu, 08 Apr 2010 21:17:39 +0200
> Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 
> > There is nothing preventing the anon_vma from being detached while we
> > are spinning to acquire the lock.
> 
> Well.  The comment there clearly implies (or states) that RCU
> protection is used to "guard against races".  If that's inaccurate
> or incomplete, can we please get it fixed?

Good point, goes together with that last comment you made.

> The whole function makes be a bit queasy.
> 
> - Fails to explain why it pulls all these party tricks to read
>   page->mapping a single time.  What code path are we defending against
>   here?

>>From what I understand we race with tear-down, anon_vma_unlock() takes
anon_vma->lock, so holding the lock pins the anon_vma.

So what we do to acquire a stable anon_vma from a page * is to, while
holding RCU read lock, very carefully read page->mapping, extract the
anon_vma and acquire the lock.

Now, the RCU usage is a tad tricky here, anon_vma uses
SLAB_DESTROY_BY_RCU, which means that the slab will be RCU freed,
however not the objects allocated from it. This means that an anon_vma
can be re-used directly after its gets freed, but the storage will
remain valid for at least a grace period after the free.

So once we do have the lock we need to revalidate that we indeed got the
anon_vma we throught we got.

So its:

  page->mapping = NULL;
  anon_vma_unlink();
    spin_lock()
    spin_unlock()
    kmem_cache_free(anon_vma);

 VS

  page_lock_anon_vma()'s trickery.

> - 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.