From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f176.google.com (mail-wi0-f176.google.com [209.85.212.176]) by kanga.kvack.org (Postfix) with ESMTP id A0F936B0255 for ; Thu, 27 Aug 2015 14:14:38 -0400 (EDT) Received: by wicne3 with SMTP id ne3so134502wic.0 for ; Thu, 27 Aug 2015 11:14:38 -0700 (PDT) Received: from mail-wi0-f174.google.com (mail-wi0-f174.google.com. [209.85.212.174]) by mx.google.com with ESMTPS id y6si18077471wix.78.2015.08.27.11.14.37 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 27 Aug 2015 11:14:37 -0700 (PDT) Received: by widdq5 with SMTP id dq5so1023243wid.1 for ; Thu, 27 Aug 2015 11:14:36 -0700 (PDT) Date: Thu, 27 Aug 2015 20:14:35 +0200 From: Michal Hocko Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust Message-ID: <20150827181434.GB29584@dhcp22.suse.cz> References: <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> <20150827163634.GD4029@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150827163634.GD4029@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Paul E. McKenney" 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 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. > > 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. -- 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