From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f182.google.com (mail-ig0-f182.google.com [209.85.213.182]) by kanga.kvack.org (Postfix) with ESMTP id EECA36B0253 for ; Wed, 26 Aug 2015 17:29:21 -0400 (EDT) Received: by igui7 with SMTP id i7so22391122igu.0 for ; Wed, 26 Aug 2015 14:29:21 -0700 (PDT) Received: from e38.co.us.ibm.com (e38.co.us.ibm.com. [32.97.110.159]) by mx.google.com with ESMTPS id 16si283763iop.167.2015.08.26.14.29.21 for (version=TLSv1 cipher=AES128-SHA bits=128/128); Wed, 26 Aug 2015 14:29:21 -0700 (PDT) Received: from /spool/local by e38.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 26 Aug 2015 15:29:20 -0600 Received: from b03cxnp08026.gho.boulder.ibm.com (b03cxnp08026.gho.boulder.ibm.com [9.17.130.18]) by d03dlp01.boulder.ibm.com (Postfix) with ESMTP id 2D7CC1FF0049 for ; Wed, 26 Aug 2015 15:20:27 -0600 (MDT) Received: from d03av04.boulder.ibm.com (d03av04.boulder.ibm.com [9.17.195.170]) by b03cxnp08026.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t7QLSYT938666362 for ; Wed, 26 Aug 2015 14:28:34 -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 t7QLTG0i019103 for ; Wed, 26 Aug 2015 15:29:17 -0600 Date: Wed, 26 Aug 2015 14:29:16 -0700 From: "Paul E. McKenney" Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust Message-ID: <20150826212916.GG11078@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com> <1439976106-137226-5-git-send-email-kirill.shutemov@linux.intel.com> <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> 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 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. 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