From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751390AbdH3GOA (ORCPT ); Wed, 30 Aug 2017 02:14:00 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:51672 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750744AbdH3GN4 (ORCPT ); Wed, 30 Aug 2017 02:13:56 -0400 Date: Wed, 30 Aug 2017 08:13:39 +0200 From: Peter Zijlstra To: Benjamin Herrenschmidt Cc: "Kirill A. Shutemov" , Laurent Dufour , paulmck@linux.vnet.ibm.com, akpm@linux-foundation.org, ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , mpe@ellerman.id.au, paulus@samba.org, Thomas Gleixner , Ingo Molnar , hpa@zytor.com, Will Deacon , linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com, npiggin@gmail.com, bsingharora@gmail.com, Tim Chen , linuxppc-dev@lists.ozlabs.org, x86@kernel.org Subject: Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure Message-ID: <20170830061339.GH32112@worktop.programming.kicks-ass.net> References: <1503007519-26777-1-git-send-email-ldufour@linux.vnet.ibm.com> <1503007519-26777-15-git-send-email-ldufour@linux.vnet.ibm.com> <20170827001823.n5wgkfq36z6snvf2@node.shutemov.name> <20170828093727.5wldedputadanssh@hirez.programming.kicks-ass.net> <1503954877.4850.19.camel@kernel.crashing.org> <20170829083352.qrsxvk3lkiydi3o2@hirez.programming.kicks-ass.net> <20170829112731.vhgwrzwwlimdbjcn@hirez.programming.kicks-ass.net> <1504041570.2358.30.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1504041570.2358.30.camel@kernel.crashing.org> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 30, 2017 at 07:19:30AM +1000, Benjamin Herrenschmidt wrote: > On Tue, 2017-08-29 at 13:27 +0200, Peter Zijlstra wrote: > > mpe helped me out and explained that is the PWC hint to TBLIE. > > > > So, you set need_flush_all when you unhook pud/pmd/pte which you then > > use to set PWC. So free_pgtables() will do the PWC when it unhooks > > higher level pages. > > > > But you're right that there's some issues, free_pgtables() itself > > doesn't seem to use mm->page_table_lock,pmd->lock _AT_ALL_ to unhook the > > pages. > > > > If it were to do that, things should work fine since those locks would > > then serialize against the speculative faults, we would never install a > > page if the VMA would be under tear-down and it would thus not be > > visible to your caches either. > > That's one case. I don't remember of *all* the cases to be honest, but > I do remember several times over the past few years thinking "ah we are > fine because the mm sem taken for writing protects us from any > concurrent tree structure change" :-) Well, installing always seems to use the locks (it needs to, because its always done with down_read()), that only leaves removal, and the only place I know that removes stuff is free_pgtables(). But I think I found another fun place, copy_page_range(). While it (pointlessly) takes all the PTLs on the dst mm it walks the src page tables without any PTLs. This means that if we have a multi-threaded process doing fork() a thread of the src mm could instantiate page-tables that will not be copied over. Of course, this is highly dubious behaviour to begin with, and I don't think there's anything fundamentally wrong with missing those pages but we should document this stuff.