From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 67D1744084A for ; Mon, 10 Jul 2017 13:48:52 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id v26so120263455pfa.0 for ; Mon, 10 Jul 2017 10:48:52 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id i1si8343483pfa.122.2017.07.10.10.48.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 10 Jul 2017 10:48:51 -0700 (PDT) Received: from pps.filterd (m0098399.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v6AHmiNa097814 for ; Mon, 10 Jul 2017 13:48:50 -0400 Received: from e06smtp14.uk.ibm.com (e06smtp14.uk.ibm.com [195.75.94.110]) by mx0a-001b2d01.pphosted.com with ESMTP id 2bmb55r1m8-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 10 Jul 2017 13:48:50 -0400 Received: from localhost by e06smtp14.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 10 Jul 2017 18:48:48 +0100 From: Laurent Dufour Subject: Re: [RFC v5 01/11] mm: Dont assume page-table invariance during faults 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> Date: Mon, 10 Jul 2017 19:48:43 +0200 MIME-Version: 1.0 In-Reply-To: <1499411222.23251.5.camel@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: Sender: owner-linux-mm@kvack.org List-ID: To: Balbir Singh , 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 Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com, npiggin@gmail.com, Tim Chen 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. -- 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