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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas of a mergeable VMA
Cc: kosaki.motohiro@jp.fujitsu.com, Rik van Riel <riel@redhat.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Borislav Petkov <bp@alien8.de>, Johannes Weiner <hannes@cmpxchg.org>,
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       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Lee Schermerhorn <Lee.Schermerhorn@hp.com>,
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       Hugh Dickins <hugh.dickins@tiscali.co.uk>, sgunderson@bigfoot.com
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Date: Tue, 13 Apr 2010 19:53:33 +0900 (JST)
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>  struct anon_vma *page_lock_anon_vma(struct page *page)
>  {
> @@ -294,14 +309,24 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
>  	unsigned long anon_mapping;
>  
>  	rcu_read_lock();
> -	anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
> +	anon_mapping = (unsigned long)rcu_dereference(page->mapping);
>  	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>  		goto out;
> -	if (!page_mapped(page))
> -		goto out;
>  
>  	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
>  	spin_lock(&anon_vma->lock);

Does anon->lock dereference is guranteed if page->_mapcount==-1?
It can be freed miliseconds ago, rcu_read_lock() doesn't provide such
gurantee.

perhaps, I'm missing your point.