Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752105AbdHHKF6 (ORCPT ); Tue, 8 Aug 2017 06:05:58 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:47654 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751921AbdHHKF5 (ORCPT ); Tue, 8 Aug 2017 06:05:57 -0400 Subject: Re: [RFC v5 01/11] mm: Dont assume page-table invariance during faults 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 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> 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 From: Anshuman Khandual Date: Tue, 8 Aug 2017 15:34:41 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable x-cbid: 17080810-0044-0000-0000-00000281DCCE X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 17080810-0045-0000-0000-000007145F5F Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2017-08-08_04:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=2 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1706020000 definitions=main-1708080161 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1725 Lines: 38 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 ?