Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753273AbbH0R26 (ORCPT ); Thu, 27 Aug 2015 13:28:58 -0400 Received: from mail-pa0-f54.google.com ([209.85.220.54]:33303 "EHLO mail-pa0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751606AbbH0R25 (ORCPT ); Thu, 27 Aug 2015 13:28:57 -0400 Date: Thu, 27 Aug 2015 10:28:48 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Michal Hocko cc: "Paul E. McKenney" , 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 Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust In-Reply-To: <20150827160355.GI27052@dhcp22.suse.cz> Message-ID: 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> <20150827160355.GI27052@dhcp22.suse.cz> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1414 Lines: 35 On Thu, 27 Aug 2015, Michal Hocko wrote: > On Thu 27-08-15 17:09:17, Michal Hocko wrote: > [...] > > Btw. Do we need the same think for page::mapping and KSM? > > I guess we are safe here because the address for mappings comes from > kmalloc and that aligned properly, right? Not quite right, in fact. Because usually the struct address_space is embedded within the struct inode (at i_data), and the struct inode embedded within the fs-dependent inode, and that's what's kmalloc'ed. What makes the mapping pointer low bits safe is include/linux/fs.h: struct address_space { ... } __attribute__((aligned(sizeof(long)))); Which we first had to add in for the cris architecture, which stumbled not on a genuine allocated address_space, but on that funny statically declared swapper_space in mm/swap_state.c. But struct anon_vma and KSM's struct stable_node (which depend on the same scheme for low bits of page->mapping) have no such alignment attribute specified: those ones are indeed relying on the kmalloc guarantee as you suppose. Does struct rcu_head have no __attribute__((aligned(whatever)))? Perhaps that attribute should be added when it's needed. Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/