Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752756AbdGKE0O (ORCPT ); Tue, 11 Jul 2017 00:26:14 -0400 Received: from mail-pf0-f196.google.com ([209.85.192.196]:34598 "EHLO mail-pf0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751077AbdGKE0N (ORCPT ); Tue, 11 Jul 2017 00:26:13 -0400 Date: Tue, 11 Jul 2017 14:26:01 +1000 From: Balbir Singh To: Laurent Dufour Cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org, akpm@linux-foundation.org, kirill@shutemov.name, ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com, npiggin@gmail.com, Tim Chen Subject: Re: [RFC v5 01/11] mm: Dont assume page-table invariance during faults Message-ID: <20170711142601.27b8fd32@firefly.ozlabs.ibm.com> In-Reply-To: References: <1497635555-25679-1-git-send-email-ldufour@linux.vnet.ibm.com> <1497635555-25679-2-git-send-email-ldufour@linux.vnet.ibm.com> <1499411222.23251.5.camel@gmail.com> X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1643 Lines: 43 On Mon, 10 Jul 2017 19:48:43 +0200 Laurent Dufour wrote: > On 07/07/2017 09:07, Balbir Singh wrote: > > On Fri, 2017-06-16 at 19:52 +0200, Laurent Dufour wrote: > >> From: Peter Zijlstra > >> > >> One of the side effects of speculating on faults (without holding > >> mmap_sem) is that we can race with free_pgtables() and therefore we > >> cannot assume the page-tables will stick around. > >> > >> Remove the relyance on the pte pointer. > > ^^ reliance > > > > Looking at the changelog and the code the impact is not clear. > > It looks like after this patch we always assume the pte is not > > the same. What is the impact of this patch? > > Hi Balbir, > > In most of the case pte_unmap_same() was returning 1, which meaning that > do_swap_page() should do its processing. > > So in most of the case there will be no impact. > > Now regarding the case where pte_unmap_safe() was returning 0, and thus > do_swap_page return 0 too, this happens when the page has already been > swapped back. This may happen before do_swap_page() get called or while in > the call to do_swap_page(). In that later case, the check done when > swapin_readahead() returns will detect that case. > > The worst case would be that a page fault is occuring on 2 threads at the > same time on the same swapped out page. In that case one thread will take > much time looping in __read_swap_cache_async(). But in the regular page > fault path, this is even worse since the thread would wait for semaphore to > be released before starting anything. > > Sounds good! Thanks, Balbir Singh