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: Wed, 16 Jan 2019 08:08:14 -0500 Message-ID: <20190116130813.GA3617@redhat.com> References: <20190111165141.GB3190@redhat.com> <1b37061c-5598-1b02-2983-80003f1c71f2@nvidia.com> <20190112020228.GA5059@redhat.com> <294bdcfa-5bf9-9c09-9d43-875e8375e264@nvidia.com> <20190112024625.GB5059@redhat.com> <20190114145447.GJ13316@quack2.suse.cz> <20190114172124.GA3702@redhat.com> <20190115080759.GC29524@quack2.suse.cz> <20190116113819.GD26069@quack2.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Cc: John Hubbard , 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: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <20190116113819.GD26069@quack2.suse.cz> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Wed, Jan 16, 2019 at 12:38:19PM +0100, Jan Kara wrote: > On Tue 15-01-19 09:07:59, Jan Kara wrote: > > Agreed. So with page lock it would actually look like: > > > > get_page_pin() > > lock_page(page); > > wait_for_stable_page(); > > atomic_add(&page->_refcount, PAGE_PIN_BIAS); > > unlock_page(page); > > > > And if we perform page_pinned() check under page lock, then if > > page_pinned() returned false, we are sure page is not and will not be > > pinned until we drop the page lock (and also until page writeback is > > completed if needed). > > After some more though, why do we even need wait_for_stable_page() and > lock_page() in get_page_pin()? > > During writepage page_mkclean() will write protect all page tables. So > there can be no new writeable GUP pins until we unlock the page as all such > GUPs will have to first go through fault and ->page_mkwrite() handler. And > that will wait on page lock and do wait_for_stable_page() for us anyway. > Am I just confused? Yeah with page lock it should synchronize on the pte but you still need to check for writeback iirc the page is unlocked after file system has queue up the write and thus the page can be unlock with write back pending (and PageWriteback() == trye) and i am not sure that in that states we can safely let anyone write to that page. I am assuming that in some case the block device also expect stable page content (RAID stuff). So the PageWriteback() test is not only for racing page_mkclean()/ test_set_page_writeback() and GUP but also for pending write back. > That actually touches on another question I wanted to get opinions on. GUP > can be for read and GUP can be for write (that is one of GUP flags). > Filesystems with page cache generally have issues only with GUP for write > as it can currently corrupt data, unexpectedly dirty page etc.. DAX & memory > hotplug have issues with both (DAX cannot truncate page pinned in any way, > memory hotplug will just loop in kernel until the page gets unpinned). So > we probably want to track both types of GUP pins and page-cache based > filesystems will take the hit even if they don't have to for read-pins? Yes the distinction between read and write would be nice. With the map count solution you can only increment the mapcount for GUP(write=true). With pin bias the issue is that a big number of read pin can trigger false positive ie you would do: GUP(vaddr, write) ... if (write) atomic_add(page->refcount, PAGE_PIN_BIAS) else atomic_inc(page->refcount) PUP(page, write) if (write) atomic_add(page->refcount, -PAGE_PIN_BIAS) else atomic_dec(page->refcount) I am guessing false positive because of too many read GUP is ok as it should be unlikely and when it happens then we take the hit. 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 A841AC43387 for ; Wed, 16 Jan 2019 13:08:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 809BB206C2 for ; Wed, 16 Jan 2019 13:08:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2392974AbfAPNIU (ORCPT ); Wed, 16 Jan 2019 08:08:20 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44362 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390247AbfAPNIU (ORCPT ); Wed, 16 Jan 2019 08:08:20 -0500 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6D50F81F0F; Wed, 16 Jan 2019 13:08:19 +0000 (UTC) Received: from redhat.com (ovpn-122-22.rdu2.redhat.com [10.10.122.22]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5AA835C22D; Wed, 16 Jan 2019 13:08:16 +0000 (UTC) Date: Wed, 16 Jan 2019 08:08:14 -0500 From: Jerome Glisse To: Jan Kara Cc: John Hubbard , 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: <20190116130813.GA3617@redhat.com> References: <20190111165141.GB3190@redhat.com> <1b37061c-5598-1b02-2983-80003f1c71f2@nvidia.com> <20190112020228.GA5059@redhat.com> <294bdcfa-5bf9-9c09-9d43-875e8375e264@nvidia.com> <20190112024625.GB5059@redhat.com> <20190114145447.GJ13316@quack2.suse.cz> <20190114172124.GA3702@redhat.com> <20190115080759.GC29524@quack2.suse.cz> <20190116113819.GD26069@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190116113819.GD26069@quack2.suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.27]); Wed, 16 Jan 2019 13:08:20 +0000 (UTC) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Message-ID: <20190116130814.RRLFoLsFOQKt6Es_4GgOCdVDWw3hxWaVH3Wg1D9tra0@z> On Wed, Jan 16, 2019 at 12:38:19PM +0100, Jan Kara wrote: > On Tue 15-01-19 09:07:59, Jan Kara wrote: > > Agreed. So with page lock it would actually look like: > > > > get_page_pin() > > lock_page(page); > > wait_for_stable_page(); > > atomic_add(&page->_refcount, PAGE_PIN_BIAS); > > unlock_page(page); > > > > And if we perform page_pinned() check under page lock, then if > > page_pinned() returned false, we are sure page is not and will not be > > pinned until we drop the page lock (and also until page writeback is > > completed if needed). > > After some more though, why do we even need wait_for_stable_page() and > lock_page() in get_page_pin()? > > During writepage page_mkclean() will write protect all page tables. So > there can be no new writeable GUP pins until we unlock the page as all such > GUPs will have to first go through fault and ->page_mkwrite() handler. And > that will wait on page lock and do wait_for_stable_page() for us anyway. > Am I just confused? Yeah with page lock it should synchronize on the pte but you still need to check for writeback iirc the page is unlocked after file system has queue up the write and thus the page can be unlock with write back pending (and PageWriteback() == trye) and i am not sure that in that states we can safely let anyone write to that page. I am assuming that in some case the block device also expect stable page content (RAID stuff). So the PageWriteback() test is not only for racing page_mkclean()/ test_set_page_writeback() and GUP but also for pending write back. > That actually touches on another question I wanted to get opinions on. GUP > can be for read and GUP can be for write (that is one of GUP flags). > Filesystems with page cache generally have issues only with GUP for write > as it can currently corrupt data, unexpectedly dirty page etc.. DAX & memory > hotplug have issues with both (DAX cannot truncate page pinned in any way, > memory hotplug will just loop in kernel until the page gets unpinned). So > we probably want to track both types of GUP pins and page-cache based > filesystems will take the hit even if they don't have to for read-pins? Yes the distinction between read and write would be nice. With the map count solution you can only increment the mapcount for GUP(write=true). With pin bias the issue is that a big number of read pin can trigger false positive ie you would do: GUP(vaddr, write) ... if (write) atomic_add(page->refcount, PAGE_PIN_BIAS) else atomic_inc(page->refcount) PUP(page, write) if (write) atomic_add(page->refcount, -PAGE_PIN_BIAS) else atomic_dec(page->refcount) I am guessing false positive because of too many read GUP is ok as it should be unlikely and when it happens then we take the hit. Cheers, Jérôme