Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756650AbbHZPjU (ORCPT ); Wed, 26 Aug 2015 11:39:20 -0400 Received: from mx2.suse.de ([195.135.220.15]:58842 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756560AbbHZPjQ (ORCPT ); Wed, 26 Aug 2015 11:39:16 -0400 Subject: Re: [PATCHv3 4/5] mm: make compound_head() robust To: "Kirill A. Shutemov" , "Paul E. McKenney" 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> <20150826150412.GA16412@node.dhcp.inet.fi> Cc: Andrew Morton , "Kirill A. Shutemov" , Hugh Dickins , Andrea Arcangeli , Dave Hansen , Johannes Weiner , Michal Hocko , David Rientjes , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Lameter From: Vlastimil Babka Message-ID: <55DDDDA1.4090407@suse.cz> Date: Wed, 26 Aug 2015 17:39:13 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <20150826150412.GA16412@node.dhcp.inet.fi> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1844 Lines: 38 On 08/26/2015 05:04 PM, Kirill A. Shutemov wrote: >>> That's >>> bad news then. It's not that we would trigger that bit when the rcu_head part of >>> the union is "active". It's that pfn scanners could inspect such page at >>> arbitrary time, see the bit 0 set (due to RCU processing) and think that it's a >>> tail page of a compound page, and interpret the rest of the pointer as a pointer >>> to the head page (to test it for flags etc). >> >> On the other hand, if you avoid scanning rcu_head structures for pages >> that are currently waiting for a grace period, no problem. RCU does >> not use the rcu_head structure at all except for during the time between >> when call_rcu() is invoked on that rcu_head structure and the time that >> the callback is invoked. >> >> Is there some other page state that indicates that the page is waiting >> for a grace period? If so, you could simply avoid testing that bit in >> that case. > > No, I don't think so. > > For compound pages most of info of its state is stored in head page (e.g. > page_count(), flags, etc). So if we examine random page (pfn scanner case) > the very first thing we want to know if we stepped on tail page. > PageTail() is what I wanted to encode in the bit... > > What if we change order of fields within rcu_head and put ->func first? Or change the order of compound_head wrt the rest? > Can we expect this pointer to have bit 0 always clear? That's probably a question whether $compiler is guaranteed to align functions on all architectures... -- 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/