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 4B7BC6B037C for ; Tue, 8 Aug 2017 06:05:57 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id j83so28328775pfe.10 for ; Tue, 08 Aug 2017 03:05:57 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id s18si604773pgd.4.2017.08.08.03.05.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 08 Aug 2017 03:05:56 -0700 (PDT) Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v78A3mFQ020174 for ; Tue, 8 Aug 2017 06:05:55 -0400 Received: from e23smtp02.au.ibm.com (e23smtp02.au.ibm.com [202.81.31.144]) by mx0a-001b2d01.pphosted.com with ESMTP id 2c785pj3ux-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 08 Aug 2017 06:05:55 -0400 Received: from localhost by e23smtp02.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 8 Aug 2017 20:05:52 +1000 Received: from d23av05.au.ibm.com (d23av05.au.ibm.com [9.190.234.119]) by d23relay09.au.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id v78A5g6K33292340 for ; Tue, 8 Aug 2017 20:05:50 +1000 Received: from d23av05.au.ibm.com (localhost [127.0.0.1]) by d23av05.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id v78A5HaR002188 for ; Tue, 8 Aug 2017 20:05:17 +1000 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> From: Anshuman Khandual Date: Tue, 8 Aug 2017 15:34:41 +0530 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Message-Id: Sender: owner-linux-mm@kvack.org List-ID: To: Laurent Dufour , 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 07/10/2017 11:18 PM, 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. Can we move the detection of swap in of the same struct page back into the page table bit earlier, ideally where pte_unmap_same() present to speed up detection for the bail out case ? -- 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