Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753489AbbHZXfF (ORCPT ); Wed, 26 Aug 2015 19:35:05 -0400 Received: from e32.co.us.ibm.com ([32.97.110.150]:52404 "EHLO e32.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751415AbbHZXfC (ORCPT ); Wed, 26 Aug 2015 19:35:02 -0400 X-Helo: d03dlp01.boulder.ibm.com X-MailFrom: paulmck@linux.vnet.ibm.com X-RcptTo: linux-kernel@vger.kernel.org Date: Wed, 26 Aug 2015 16:34:56 -0700 From: "Paul E. McKenney" To: Hugh Dickins Cc: Vlastimil Babka , "Kirill A. Shutemov" , Andrew Morton , "Kirill A. Shutemov" , Andrea Arcangeli , Dave Hansen , Johannes Weiner , Michal Hocko , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Lameter Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust Message-ID: <20150826233455.GH11078@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20150820163643.dd87de0c1a73cb63866b2914@linux-foundation.org> <20150821121028.GB12016@node.dhcp.inet.fi> <55DC550D.5060501@suse.cz> <20150825183354.GC4881@node.dhcp.inet.fi> <20150825201113.GK11078@linux.vnet.ibm.com> <55DCD434.9000704@suse.cz> <20150825211954.GN11078@linux.vnet.ibm.com> <20150826212916.GG11078@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15082623-0005-0000-0000-00001793F76C Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3005 Lines: 63 On Wed, Aug 26, 2015 at 03:28:39PM -0700, Hugh Dickins wrote: > On Wed, 26 Aug 2015, Paul E. McKenney wrote: > > On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote: > > > On Tue, 25 Aug 2015, Paul E. McKenney wrote: > > > > On Tue, Aug 25, 2015 at 10:46:44PM +0200, Vlastimil Babka wrote: > > > > > On 25.8.2015 22:11, Paul E. McKenney wrote: > > > > > > On Tue, Aug 25, 2015 at 09:33:54PM +0300, Kirill A. Shutemov wrote: > > > > > >> On Tue, Aug 25, 2015 at 01:44:13PM +0200, Vlastimil Babka wrote: > > > > > >>> On 08/21/2015 02:10 PM, Kirill A. Shutemov wrote: > > > > > >>>> On Thu, Aug 20, 2015 at 04:36:43PM -0700, Andrew Morton wrote: > > > > > >>>>> On Wed, 19 Aug 2015 12:21:45 +0300 "Kirill A. Shutemov" wrote: > > > > > >>>>> > > > > > >>>>>> The patch introduces page->compound_head into third double word block in > > > > > >>>>>> front of compound_dtor and compound_order. That means it shares storage > > > > > >>>>>> space with: > > > > > >>>>>> > > > > > >>>>>> - page->lru.next; > > > > > >>>>>> - page->next; > > > > > >>>>>> - page->rcu_head.next; > > > > > >>>>>> - page->pmd_huge_pte; > > > > > >>>>>> > > > > > >>> > > > > > >>> We should probably ask Paul about the chances that rcu_head.next would like > > > > > >>> to use the bit too one day? > > > > > >> > > > > > >> +Paul. > > > > > > > > > > > > The call_rcu() function does stomp that bit, but if you stop using that > > > > > > bit before you invoke call_rcu(), no problem. > > > > > > > > > > You mean that it sets the bit 0 of rcu_head.next during its processing? > > > > > > > > Not at the moment, though RCU will splat if given a misaligned rcu_head > > > > structure because of the possibility to use that bit to flag callbacks > > > > that do nothing but free memory. If RCU needs to do that (e.g., to > > > > promote energy efficiency), then that bit might well be set during > > > > RCU grace-period processing. > > > > > > But if you do one day implement that, wouldn't sl?b.c have to use > > > call_rcu_with_added_meaning() instead of call_rcu(), to be in danger > > > of getting that bit set? (No rcu_head is placed in a PageTail page.) > > > > Good point, call_rcu_lazy(), but yes. > > > > > So although it might be a little strange not to use a variant intended > > > for freeing memory when indeed that's what it's doing, it would not be > > > the end of the world for SLAB_DESTROY_BY_RCU to carry on using straight > > > call_rcu(), in defence of the struct page safety Kirill is proposing. > > > > As long as you are OK with the bottom bit being zero throughout the RCU > > processing, yes. > > That's exactly what we want: sounds like we have no problem, thanks Paul. Whew! ;-) Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/