From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jerome Glisse Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions Date: Fri, 11 Jan 2019 21:46:25 -0500 Message-ID: <20190112024625.GB5059@redhat.com> References: <20181219020723.GD4347@redhat.com> <20181219110856.GA18345@quack2.suse.cz> <20190103015533.GA15619@redhat.com> <20190103092654.GA31370@quack2.suse.cz> <20190103144405.GC3395@redhat.com> <20190111165141.GB3190@redhat.com> <1b37061c-5598-1b02-2983-80003f1c71f2@nvidia.com> <20190112020228.GA5059@redhat.com> <294bdcfa-5bf9-9c09-9d43-875e8375e264@nvidia.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Cc: Jan Kara , Matthew Wilcox , Dave Chinner , Dan Williams , John Hubbard , Andrew Morton , Linux MM , tom@talpey.com, Al Viro , benve@cisco.com, Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , mike.marciniszyn@intel.com, rcampbell@nvidia.com, Linux Kernel Mailing List , linux-fsdevel To: John Hubbard Return-path: Content-Disposition: inline In-Reply-To: <294bdcfa-5bf9-9c09-9d43-875e8375e264@nvidia.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Fri, Jan 11, 2019 at 06:38:44PM -0800, John Hubbard wrote: > On 1/11/19 6:02 PM, Jerome Glisse wrote: > > On Fri, Jan 11, 2019 at 05:04:05PM -0800, John Hubbard wrote: > >> On 1/11/19 8:51 AM, Jerome Glisse wrote: > >>> On Thu, Jan 10, 2019 at 06:59:31PM -0800, John Hubbard wrote: > >>>> On 1/3/19 6:44 AM, Jerome Glisse wrote: > >>>>> On Thu, Jan 03, 2019 at 10:26:54AM +0100, Jan Kara wrote: > >>>>>> On Wed 02-01-19 20:55:33, Jerome Glisse wrote: > >>>>>>> On Wed, Dec 19, 2018 at 12:08:56PM +0100, Jan Kara wrote: > >>>>>>>> On Tue 18-12-18 21:07:24, Jerome Glisse wrote: > >>>>>>>>> On Tue, Dec 18, 2018 at 03:29:34PM -0800, John Hubbard wrote: > >>> [...] > >> > >> Hi Jerome, > >> > >> Looks good, in a conceptual sense. Let me do a brain dump of how I see it, > >> in case anyone spots a disastrous conceptual error (such as the lock_page > >> point), while I'm putting together the revised patchset. > >> > >> I've studied this carefully, and I agree that using mapcount in > >> this way is viable, *as long* as we use a lock (or a construct that looks just > >> like one: your "memory barrier, check, retry" is really just a lock) in > >> order to hold off gup() while page_mkclean() is in progress. In other words, > >> nothing that increments mapcount may proceed while page_mkclean() is running. > > > > No, increment to page->_mapcount are fine while page_mkclean() is running. > > The above solution do work no matter what happens thanks to the memory > > barrier. By clearing the pin flag first and reading the page->_mapcount > > after (and doing the reverse in GUP) we know that a racing GUP will either > > have its pin page clear but the incremented mapcount taken into account by > > page_mkclean() or page_mkclean() will miss the incremented mapcount but > > it will also no clear the pin flag set concurrently by any GUP. > > > > Here are all the possible time line: > > [T1]: > > GUP on CPU0 | page_mkclean() on CPU1 > > | > > [G2] atomic_inc(&page->mapcount) | > > [G3] smp_wmb(); | > > [G4] SetPagePin(page); | > > ... > > | [C1] pined = TestClearPagePin(page); > > It appears that you're using the "page pin is clear" to indicate that > page_mkclean() is running. The problem is, that approach leads to toggling > the PagePin flag, and so an observer (other than gup or page_mkclean) will > see intervals during which the PagePin flag is clear, when conceptually it > should be set. > > Jan and other FS people, is it definitely the case that we only have to take > action (defer, wait, revoke, etc) for gup-pinned pages, in page_mkclean()? > Because I recall from earlier experiments that there were several places, not > just page_mkclean(). Yes and it is fine to temporarily have the pin flag unstable. Anything that need stable page content will have to lock the page so will have to sync against any page_mkclean() and in the end the only thing were we want to check the pin flag is when doing write back ie after page_mkclean() while the page is still locked. If they are any other place that need to check the pin flag then they will need to lock the page. But i can not think of any other place right now. [...] > >> The other idea that you and Dan (and maybe others) pointed out was a debug > >> option, which we'll certainly need in order to safely convert all the call > >> sites. (Mirror the mappings at a different kernel offset, so that put_page() > >> and put_user_page() can verify that the right call was made.) That will be > >> a separate patchset, as you recommended. > >> > >> I'll even go as far as recommending the page lock itself. I realize that this > >> adds overhead to gup(), but we *must* hold off page_mkclean(), and I believe > >> that this (below) has similar overhead to the notes above--but is *much* easier > >> to verify correct. (If the page lock is unacceptable due to being so widely used, > >> then I'd recommend using another page bit to do the same thing.) > > > > Please page lock is pointless and it will not work for GUP fast. The above > > scheme do work and is fine. I spend the day again thinking about all memory > > ordering and i do not see any issues. > > > > Why is it that page lock cannot be used for gup fast, btw? Well it can not happen within the preempt disable section. But after as a post pass before GUP_fast return and after reenabling preempt then it is fine like it would be for regular GUP. But locking page for GUP is also likely to slow down some workload (with direct-IO). Cheers, Jérôme From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E283FC43387 for ; Sat, 12 Jan 2019 02:46:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BC4512084E for ; Sat, 12 Jan 2019 02:46:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726502AbfALCqb (ORCPT ); Fri, 11 Jan 2019 21:46:31 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38952 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726433AbfALCqb (ORCPT ); Fri, 11 Jan 2019 21:46:31 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6E672C057F37; Sat, 12 Jan 2019 02:46:30 +0000 (UTC) Received: from redhat.com (ovpn-120-63.rdu2.redhat.com [10.10.120.63]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 787A31019627; Sat, 12 Jan 2019 02:46:27 +0000 (UTC) Date: Fri, 11 Jan 2019 21:46:25 -0500 From: Jerome Glisse To: John Hubbard Cc: Jan Kara , Matthew Wilcox , Dave Chinner , Dan Williams , John Hubbard , Andrew Morton , Linux MM , tom@talpey.com, Al Viro , benve@cisco.com, Christoph Hellwig , Christopher Lameter , "Dalessandro, Dennis" , Doug Ledford , Jason Gunthorpe , Michal Hocko , mike.marciniszyn@intel.com, rcampbell@nvidia.com, Linux Kernel Mailing List , linux-fsdevel Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions Message-ID: <20190112024625.GB5059@redhat.com> References: <20181219020723.GD4347@redhat.com> <20181219110856.GA18345@quack2.suse.cz> <20190103015533.GA15619@redhat.com> <20190103092654.GA31370@quack2.suse.cz> <20190103144405.GC3395@redhat.com> <20190111165141.GB3190@redhat.com> <1b37061c-5598-1b02-2983-80003f1c71f2@nvidia.com> <20190112020228.GA5059@redhat.com> <294bdcfa-5bf9-9c09-9d43-875e8375e264@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <294bdcfa-5bf9-9c09-9d43-875e8375e264@nvidia.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Sat, 12 Jan 2019 02:46:30 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Message-ID: <20190112024625.8PyJrKcGih7bom3fPBHgatWbWv9Cbskm9s5j0fSL3f8@z> On Fri, Jan 11, 2019 at 06:38:44PM -0800, John Hubbard wrote: > On 1/11/19 6:02 PM, Jerome Glisse wrote: > > On Fri, Jan 11, 2019 at 05:04:05PM -0800, John Hubbard wrote: > >> On 1/11/19 8:51 AM, Jerome Glisse wrote: > >>> On Thu, Jan 10, 2019 at 06:59:31PM -0800, John Hubbard wrote: > >>>> On 1/3/19 6:44 AM, Jerome Glisse wrote: > >>>>> On Thu, Jan 03, 2019 at 10:26:54AM +0100, Jan Kara wrote: > >>>>>> On Wed 02-01-19 20:55:33, Jerome Glisse wrote: > >>>>>>> On Wed, Dec 19, 2018 at 12:08:56PM +0100, Jan Kara wrote: > >>>>>>>> On Tue 18-12-18 21:07:24, Jerome Glisse wrote: > >>>>>>>>> On Tue, Dec 18, 2018 at 03:29:34PM -0800, John Hubbard wrote: > >>> [...] > >> > >> Hi Jerome, > >> > >> Looks good, in a conceptual sense. Let me do a brain dump of how I see it, > >> in case anyone spots a disastrous conceptual error (such as the lock_page > >> point), while I'm putting together the revised patchset. > >> > >> I've studied this carefully, and I agree that using mapcount in > >> this way is viable, *as long* as we use a lock (or a construct that looks just > >> like one: your "memory barrier, check, retry" is really just a lock) in > >> order to hold off gup() while page_mkclean() is in progress. In other words, > >> nothing that increments mapcount may proceed while page_mkclean() is running. > > > > No, increment to page->_mapcount are fine while page_mkclean() is running. > > The above solution do work no matter what happens thanks to the memory > > barrier. By clearing the pin flag first and reading the page->_mapcount > > after (and doing the reverse in GUP) we know that a racing GUP will either > > have its pin page clear but the incremented mapcount taken into account by > > page_mkclean() or page_mkclean() will miss the incremented mapcount but > > it will also no clear the pin flag set concurrently by any GUP. > > > > Here are all the possible time line: > > [T1]: > > GUP on CPU0 | page_mkclean() on CPU1 > > | > > [G2] atomic_inc(&page->mapcount) | > > [G3] smp_wmb(); | > > [G4] SetPagePin(page); | > > ... > > | [C1] pined = TestClearPagePin(page); > > It appears that you're using the "page pin is clear" to indicate that > page_mkclean() is running. The problem is, that approach leads to toggling > the PagePin flag, and so an observer (other than gup or page_mkclean) will > see intervals during which the PagePin flag is clear, when conceptually it > should be set. > > Jan and other FS people, is it definitely the case that we only have to take > action (defer, wait, revoke, etc) for gup-pinned pages, in page_mkclean()? > Because I recall from earlier experiments that there were several places, not > just page_mkclean(). Yes and it is fine to temporarily have the pin flag unstable. Anything that need stable page content will have to lock the page so will have to sync against any page_mkclean() and in the end the only thing were we want to check the pin flag is when doing write back ie after page_mkclean() while the page is still locked. If they are any other place that need to check the pin flag then they will need to lock the page. But i can not think of any other place right now. [...] > >> The other idea that you and Dan (and maybe others) pointed out was a debug > >> option, which we'll certainly need in order to safely convert all the call > >> sites. (Mirror the mappings at a different kernel offset, so that put_page() > >> and put_user_page() can verify that the right call was made.) That will be > >> a separate patchset, as you recommended. > >> > >> I'll even go as far as recommending the page lock itself. I realize that this > >> adds overhead to gup(), but we *must* hold off page_mkclean(), and I believe > >> that this (below) has similar overhead to the notes above--but is *much* easier > >> to verify correct. (If the page lock is unacceptable due to being so widely used, > >> then I'd recommend using another page bit to do the same thing.) > > > > Please page lock is pointless and it will not work for GUP fast. The above > > scheme do work and is fine. I spend the day again thinking about all memory > > ordering and i do not see any issues. > > > > Why is it that page lock cannot be used for gup fast, btw? Well it can not happen within the preempt disable section. But after as a post pass before GUP_fast return and after reenabling preempt then it is fine like it would be for regular GUP. But locking page for GUP is also likely to slow down some workload (with direct-IO). Cheers, Jérôme