From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f174.google.com (mail-qk0-f174.google.com [209.85.220.174]) by kanga.kvack.org (Postfix) with ESMTP id 278CE6B0254 for ; Thu, 27 Aug 2015 12:36:43 -0400 (EDT) Received: by qkfh127 with SMTP id h127so13482768qkf.1 for ; Thu, 27 Aug 2015 09:36:43 -0700 (PDT) Received: from e31.co.us.ibm.com (e31.co.us.ibm.com. [32.97.110.149]) by mx.google.com with ESMTPS id f23si3098194qge.2.2015.08.27.09.36.42 for (version=TLSv1 cipher=AES128-SHA bits=128/128); Thu, 27 Aug 2015 09:36:42 -0700 (PDT) Received: from /spool/local by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 27 Aug 2015 10:36:41 -0600 Received: from b03cxnp08027.gho.boulder.ibm.com (b03cxnp08027.gho.boulder.ibm.com [9.17.130.19]) by d03dlp03.boulder.ibm.com (Postfix) with ESMTP id 4C62F19D8040 for ; Thu, 27 Aug 2015 10:27:33 -0600 (MDT) Received: from d03av04.boulder.ibm.com (d03av04.boulder.ibm.com [9.17.195.170]) by b03cxnp08027.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t7RGabe645088920 for ; Thu, 27 Aug 2015 09:36:37 -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 t7RGaa1q018298 for ; Thu, 27 Aug 2015 10:36:37 -0600 Date: Thu, 27 Aug 2015 09:36:34 -0700 From: "Paul E. McKenney" Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust Message-ID: <20150827163634.GD4029@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> <20150827150917.GF27052@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150827150917.GF27052@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Hugh Dickins , Vlastimil Babka , "Kirill A. Shutemov" , Andrew Morton , "Kirill A. Shutemov" , Andrea Arcangeli , Dave Hansen , Johannes Weiner , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Lameter On Thu, Aug 27, 2015 at 05:09:17PM +0200, Michal Hocko wrote: > On Wed 26-08-15 14:29:16, Paul E. McKenney wrote: > > On Wed, Aug 26, 2015 at 11:18:45AM -0700, Hugh Dickins wrote: > [...] > > > 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. > > I am really not sure I udnerstand. What will prevent > call_rcu(&page->rcu_head, free_page_rcu) done in a random driver? As long as it uses call_rcu(), call_rcu_bh(), call_rcu_sched(), or call_srcu() and not some future call_rcu_lazy(), no problem. But yes, if you are going to assume that RCU leaves the bottom bit of the rcu_head structure's ->next field zero, then everything everywhere in the kernel might in the future need to be careful of exactly what variant of call_rcu() is used. > Cannot the RCU simply claim bit1? I can see 1146edcbef37 ("rcu: Loosen > __call_rcu()'s rcu_head alignment constraint") but AFAIU all it would > take to fix this would be to require struct rcu_head to be aligned to > 32b no? There are some architectures that guarantee only 16-bit alignment. If those architectures are fixed to do 32-bit alignment, or if support for them is dropped, then the future restrictions mentioned above could be dropped. Thanx, Paul > Btw. Do we need the same think for page::mapping and KSM? > -- > Michal Hocko > SUSE Labs > -- 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