From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f176.google.com (mail-io0-f176.google.com [209.85.223.176]) by kanga.kvack.org (Postfix) with ESMTP id 8079C6B0253 for ; Thu, 27 Aug 2015 15:08:17 -0400 (EDT) Received: by iodt126 with SMTP id t126so68820742iod.2 for ; Thu, 27 Aug 2015 12:08:17 -0700 (PDT) Received: from e39.co.us.ibm.com (e39.co.us.ibm.com. [32.97.110.160]) by mx.google.com with ESMTPS id z21si2779153iod.138.2015.08.27.12.08.16 for (version=TLSv1 cipher=AES128-SHA bits=128/128); Thu, 27 Aug 2015 12:08:16 -0700 (PDT) Received: from /spool/local by e39.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 27 Aug 2015 13:08:16 -0600 Received: from b03cxnp07029.gho.boulder.ibm.com (b03cxnp07029.gho.boulder.ibm.com [9.17.130.16]) by d03dlp03.boulder.ibm.com (Postfix) with ESMTP id 2B36019D8040 for ; Thu, 27 Aug 2015 12:59:10 -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 t7RJ8E5F49414368 for ; Thu, 27 Aug 2015 12:08:14 -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 t7RJ8CC2027696 for ; Thu, 27 Aug 2015 13:08:14 -0600 Date: Thu, 27 Aug 2015 12:01:38 -0700 From: "Paul E. McKenney" Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust Message-ID: <20150827190138.GG4029@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <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> <20150827163634.GD4029@linux.vnet.ibm.com> <20150827181434.GB29584@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150827181434.GB29584@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 08:14:35PM +0200, Michal Hocko wrote: > On Thu 27-08-15 09:36:34, Paul E. McKenney wrote: > > 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. > > OK, so it would be call_rcu_$special to use the bit. This wasn't entirely > clear to me. I thought it would be opposite. Yes. And I cannot resist adding that the need to avoid call_rcu_$special() would be with respect to a given rcu_head structure, not global. Though I believe that you already figured that out. ;-) > > > 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. > > My understanding of the discussion which led to the above patch is that > m68k allows for 32b alignment you just have to be explicit about that > (http://thread.gmane.org/gmane.linux.ports.m68k/5932/focus=5960). Which > other archs would be affected? > > I mean, this patch allows for quite some simplification in the mm code. > And I think that RCU can live with mm of the low bits without any > issues. You've said that one bit should be sufficient for the RCU use > case. So having 2 bits sounds like a good thing. As long as MM doesn't use call_rcu_$special() for the rcu_head structure in question, as long as MM is OK with the bottom bit of ->next always being zero during a grace period, and as long as MM avoids writing to ->next during a grace period, we should be good as is, even if a call_rcu_$special() becomes necessary. 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