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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>, Ingo Molnar <mingo@elte.hu>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-mm@kvack.org,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	David Miller <davem@davemloft.net>,
	Hugh Dickins <hugh.dickins@tiscali.co.uk>,
	Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@kernel.dk>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@linux.intel.com>
Subject: Re: [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path
Date: Wed, 20 Apr 2011 17:00:29 +0200	[thread overview]
Message-ID: <1303311629.8345.230.camel@twins> (raw)
In-Reply-To: <1303303124.8345.218.camel@twins>

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


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WARNING: multiple messages have this Message-ID (diff)
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>, Ingo Molnar <mingo@elte.hu>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-mm@kvack.org,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	David Miller <davem@davemloft.net>,
	Hugh Dickins <hugh.dickins@tiscali.co.uk>,
	Mel Gorman <mel@csn.ul.ie>, Nick Piggin <npiggin@kernel.dk>,
	Paul McKenney <paulmck@linux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@linux.intel.com>
Subject: Re: [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path
Date: Wed, 20 Apr 2011 17:00:29 +0200	[thread overview]
Message-ID: <1303311629.8345.230.camel@twins> (raw)
Message-ID: <20110420150029.OEwQfxc4FClW_vJE-9NFS0ZutT7PG-Mykr3CM_3TjcY@z> (raw)
In-Reply-To: <1303303124.8345.218.camel@twins>

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



  parent reply	other threads:[~2011-04-20 15:00 UTC|newest]

Thread overview: 79+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-04-01 12:12 [PATCH 00/20] mm: Preemptibility -v10 Peter Zijlstra
2011-04-01 12:12 ` Peter Zijlstra
2011-04-01 12:12 ` [PATCH 01/20] mm: mmu_gather rework Peter Zijlstra
2011-04-01 12:12   ` Peter Zijlstra
2011-04-19 20:06   ` Andrew Morton
2011-04-19 20:06     ` Andrew Morton
2011-04-20  8:47     ` Peter Zijlstra
2011-04-20  8:47       ` Peter Zijlstra
2011-04-20  9:10       ` Peter Zijlstra
2011-04-20  9:10         ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 02/20] powerpc: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 03/20] sparc: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 04/20] s390: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 05/20] arm: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 06/20] sh: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 07/20] ia64: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 08/20] um: " Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 09/20] mm: Now that all old mmu_gather code is gone, remove the storage Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 10/20] mm, powerpc: Move the RCU page-table freeing into generic code Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 11/20] s390: use generic RCP page-table freeing Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 12/20] mm: Extended batches for generic mmu_gather Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-19 20:06   ` Andrew Morton
2011-04-19 20:06     ` Andrew Morton
2011-04-20 10:40     ` Peter Zijlstra
2011-04-20 10:40       ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 13/20] lockdep, mutex: Provide mutex_lock_nest_lock Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-19 20:06   ` Andrew Morton
2011-04-19 20:06     ` Andrew Morton
2011-04-20 11:03     ` Peter Zijlstra
2011-04-20 11:03       ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 14/20] mm: Remove i_mmap_lock lockbreak Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-19 20:07   ` Andrew Morton
2011-04-19 20:07     ` Andrew Morton
2011-04-21 13:32     ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 15/20] mm: Convert i_mmap_lock to a mutex Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-19 20:07   ` Andrew Morton
2011-04-19 20:07     ` Andrew Morton
2011-04-21 13:28     ` Peter Zijlstra
2011-04-21 13:28       ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 16/20] mm: Revert page_lock_anon_vma() lock annotation Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 17/20] mm: Improve page_lock_anon_vma() comment Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 18/20] mm: Use refcounts for page_lock_anon_vma() Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 19/20] mm: Convert anon_vma->lock to a mutex Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-19 20:07   ` Andrew Morton
2011-04-19 20:07     ` Andrew Morton
2011-04-21 13:28     ` Peter Zijlstra
2011-04-21 13:28       ` Peter Zijlstra
2011-04-01 12:13 ` [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path Peter Zijlstra
2011-04-01 12:13   ` Peter Zijlstra
2011-04-19 20:08   ` Andrew Morton
2011-04-19 20:08     ` Andrew Morton
2011-04-20 12:38     ` Peter Zijlstra
2011-04-20 12:38       ` Peter Zijlstra
2011-04-20 15:00       ` Peter Zijlstra [this message]
2011-04-20 15:00         ` Peter Zijlstra
2011-04-01 13:51 ` [PATCH 00/20] mm: Preemptibility -v10 Peter Zijlstra
2011-04-01 13:51   ` Peter Zijlstra
  -- strict thread matches above, loose matches on Subject: below --
2010-10-18 11:24 [PATCH 00/20] mm: Preemptibility -v5 Peter Zijlstra
2010-10-18 11:24 ` [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path Peter Zijlstra
2010-10-18 11:24   ` Peter Zijlstra
2010-08-28 14:16 [PATCH 00/20] mm: Preemptibility -v4 Peter Zijlstra
2010-08-28 14:16 ` [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path Peter Zijlstra
2010-08-28 14:16   ` Peter Zijlstra

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