From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f199.google.com (mail-qt0-f199.google.com [209.85.216.199]) by kanga.kvack.org (Postfix) with ESMTP id 9A755280281 for ; Wed, 17 Jan 2018 03:57:29 -0500 (EST) Received: by mail-qt0-f199.google.com with SMTP id z37so3078518qtj.15 for ; Wed, 17 Jan 2018 00:57:29 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id p60si4003053qtd.169.2018.01.17.00.57.28 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 17 Jan 2018 00:57:28 -0800 (PST) Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w0H8uSWR064712 for ; Wed, 17 Jan 2018 03:57:27 -0500 Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) by mx0a-001b2d01.pphosted.com with ESMTP id 2fj1v9v1w2-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 17 Jan 2018 03:57:27 -0500 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 17 Jan 2018 08:57:24 -0000 Subject: Re: [PATCH v6 03/24] mm: Dont assume page-table invariance during faults References: <1515777968-867-1-git-send-email-ldufour@linux.vnet.ibm.com> <1515777968-867-4-git-send-email-ldufour@linux.vnet.ibm.com> <87d129tccz.fsf@linux.intel.com> From: Laurent Dufour Date: Wed, 17 Jan 2018 09:57:14 +0100 MIME-Version: 1.0 In-Reply-To: <87d129tccz.fsf@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <476660c5-771c-0125-7d04-0e5a8d8bf65d@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Andi Kleen Cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org, akpm@linux-foundation.org, kirill@shutemov.name, mhocko@kernel.org, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org, Thomas Gleixner , Ingo Molnar , hpa@zytor.com, Will Deacon , Sergey Senozhatsky , Andrea Arcangeli , Alexei Starovoitov , kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com, 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 On 17/01/2018 04:04, Andi Kleen wrote: > Laurent Dufour writes: > >> 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 reliance on the pte pointer. > > This needs a lot more explanation. So why is this code not needed with > SPF only? Hi Andi, This is a good question, and I should detail that more in the commit's log. Here is my response to Balbir when he asked for: On 10/07/2017 19:48, 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. > > Cheers, > Laurent. > I'll add that to the commit's log. Thanks, Laurent. -- 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