From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with ESMTP id ED8898D003B for ; Wed, 20 Apr 2011 11:01:10 -0400 (EDT) Subject: Re: [PATCH 20/20] mm: Optimize page_lock_anon_vma() fast-path From: Peter Zijlstra In-Reply-To: <1303303124.8345.218.camel@twins> References: <20110401121258.211963744@chello.nl> <20110401121726.285750519@chello.nl> <20110419130800.7148a602.akpm@linux-foundation.org> <1303303124.8345.218.camel@twins> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Date: Wed, 20 Apr 2011 17:00:29 +0200 Message-ID: <1303311629.8345.230.camel@twins> Mime-Version: 1.0 Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Andrea Arcangeli , Avi Kivity , Thomas Gleixner , Rik van Riel , Ingo Molnar , Linus Torvalds , linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, Benjamin Herrenschmidt , David Miller , Hugh Dickins , Mel Gorman , Nick Piggin , Paul McKenney , Yanmin Zhang On Wed, 2011-04-20 at 14:38 +0200, Peter Zijlstra wrote: > > > + if (!page_mapped(page)) { > > > + put_anon_vma(anon_vma); > > > + anon_vma =3D NULL; > > > + goto out; > > > + } > >=20 > > Also quite opaque, needs decent commentary. > >=20 > > I'd have expected this test to occur after the lock was acquired. >=20 > 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(). >=20 > So yes, I think I'll move it down because that'll widen the scope of > this optimization.=20 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org