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Subject: Re: [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path
From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>, Avi Kivity <avi@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, Rik van Riel <riel@redhat.com>,
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        Hugh Dickins <hugh.dickins@tiscali.co.uk>, Mel Gorman <mel@csn.ul.ie>,
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        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Yanmin Zhang <yanmin_zhang@linux.intel.com>
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Date: Wed, 20 Apr 2011 17:00:29 +0200
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On Wed, 2011-04-20 at 14:38 +0200, Peter Zijlstra wrote:
> > > +   if (!page_mapped(page)) {
> > > +           put_anon_vma(anon_vma);
> > > +           anon_vma = NULL;
> > > +           goto out;
> > > +   }
> > 
> > Also quite opaque, needs decent commentary.
> > 
> > I'd have expected this test to occur after the lock was acquired.
> 
> Right, so I think we could drop that test from both here and
> page_get_anon_vma() and nothing would break, its simply avoiding some
> work in case we do detect the race with page_remove_rmap().
> 
> So yes, I think I'll move it down because that'll widen the scope of
> this optimization. 

OK, so I went trawling through the linux-mm logs and actually found why
this is needed under rcu_read_lock(), sadly I cannot seem to find a web
reference to 2006 linux-mm emails.

It was Hugh who noted that page_remove_rmap() only decrements
page->_mapcount but does not clear page->mapping, therefore we must test
page_mapped() after reading page->mapping while under the
rcu_read_lock() in order to determine if the pointer obtained is still
valid.

The comment that exists today in __page_lock_anon_vma() actually tries
to explain this