From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qg0-f53.google.com (mail-qg0-f53.google.com [209.85.192.53]) by kanga.kvack.org (Postfix) with ESMTP id 7DC3D6B0253 for ; Wed, 26 Aug 2015 19:35:01 -0400 (EDT) Received: by qgeg42 with SMTP id g42so1818513qge.1 for ; Wed, 26 Aug 2015 16:35:01 -0700 (PDT) Received: from e35.co.us.ibm.com (e35.co.us.ibm.com. [32.97.110.153]) by mx.google.com with ESMTPS id 71si286326qky.1.2015.08.26.16.35.00 for (version=TLSv1 cipher=AES128-SHA bits=128/128); Wed, 26 Aug 2015 16:35:00 -0700 (PDT) Received: from /spool/local by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 26 Aug 2015 17:34:59 -0600 Received: from b03cxnp07029.gho.boulder.ibm.com (b03cxnp07029.gho.boulder.ibm.com [9.17.130.16]) by d03dlp02.boulder.ibm.com (Postfix) with ESMTP id 9A3323E4003F for ; Wed, 26 Aug 2015 17:34:57 -0600 (MDT) Received: from d03av04.boulder.ibm.com (d03av04.boulder.ibm.com [9.17.195.170]) by b03cxnp07029.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t7QNYvfV44499120 for ; Wed, 26 Aug 2015 16:34:57 -0700 Received: from d03av04.boulder.ibm.com (loopback [127.0.0.1]) by d03av04.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t7QNYuFk016683 for ; Wed, 26 Aug 2015 17:34:57 -0600 Date: Wed, 26 Aug 2015 16:34:56 -0700 From: "Paul E. McKenney" 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: Sender: owner-linux-mm@kvack.org List-ID: 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 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, 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/ . Don't email: email@kvack.org