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=ham 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 76937C43387 for ; Mon, 17 Dec 2018 19:48:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 3BF162133F for ; Mon, 17 Dec 2018 19:48:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1733256AbeLQTsH (ORCPT ); Mon, 17 Dec 2018 14:48:07 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56336 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726660AbeLQTsG (ORCPT ); Mon, 17 Dec 2018 14:48:06 -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 312C346263; Mon, 17 Dec 2018 19:48:05 +0000 (UTC) Received: from redhat.com (ovpn-125-170.rdu2.redhat.com [10.10.125.170]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 390741001F50; Mon, 17 Dec 2018 19:48:02 +0000 (UTC) Date: Mon, 17 Dec 2018 14:48:00 -0500 From: Jerome Glisse To: Matthew Wilcox Cc: Dave Chinner , Jan Kara , John Hubbard , 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: <20181217194759.GB3341@redhat.com> References: <20181207191620.GD3293@redhat.com> <3c4d46c0-aced-f96f-1bf3-725d02f11b60@nvidia.com> <20181208022445.GA7024@redhat.com> <20181210102846.GC29289@quack2.suse.cz> <20181212150319.GA3432@redhat.com> <20181212214641.GB29416@dastard> <20181214154321.GF8896@quack2.suse.cz> <20181216215819.GC10644@dastard> <20181217181148.GA3341@redhat.com> <20181217183443.GO10600@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20181217183443.GO10600@bombadil.infradead.org> 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 [10.5.110.29]); Mon, 17 Dec 2018 19:48:05 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 17, 2018 at 10:34:43AM -0800, Matthew Wilcox wrote: > On Mon, Dec 17, 2018 at 01:11:50PM -0500, Jerome Glisse wrote: > > On Mon, Dec 17, 2018 at 08:58:19AM +1100, Dave Chinner wrote: > > > Sure, that's a possibility, but that doesn't close off any race > > > conditions because there can be DMA into the page in progress while > > > the page is being bounced, right? AFAICT this ext3+DIF/DIX case is > > > different in that there is no 3rd-party access to the page while it > > > is under IO (ext3 arbitrates all access to it's metadata), and so > > > nothing can actually race for modification of the page between > > > submission and bouncing at the block layer. > > > > > > In this case, the moment the page is unlocked, anyone else can map > > > it and start (R)DMA on it, and that can happen before the bio is > > > bounced by the block layer. So AFAICT, block layer bouncing doesn't > > > solve the problem of racing writeback and DMA direct to the page we > > > are doing IO on. Yes, it reduces the race window substantially, but > > > it doesn't get rid of it. > > > > So the event flow is: > > - userspace create object that match a range of virtual address > > against a given kernel sub-system (let's say infiniband) and > > let's assume that the range is an mmap() of a regular file > > - device driver do GUP on the range (let's assume it is a write > > GUP) so if the page is not already map with write permission > > in the page table than a page fault is trigger and page_mkwrite > > happens > > - Once GUP return the page to the device driver and once the > > device driver as updated the hardware states to allow access > > to this page then from that point on hardware can write to the > > page at _any_ time, it is fully disconnected from any fs event > > like write back, it fully ignore things like page_mkclean > > > > This is how it is to day, we allowed people to push upstream such > > users of GUP. This is a fact we have to live with, we can not stop > > hardware access to the page, we can not force the hardware to follow > > page_mkclean and force a page_mkwrite once write back ends. This is > > the situation we are inheriting (and i am personnaly not happy with > > that). > > > > >From my point of view we are left with 2 choices: > > [C1] break all drivers that do not abide by the page_mkclean and > > page_mkwrite > > [C2] mitigate as much as possible the issue > > > > For [C2] the idea is to keep track of GUP per page so we know if we > > can expect the page to be written to at any time. Here is the event > > flow: > > - driver GUP the page and program the hardware, page is mark as > > GUPed > > ... > > - write back kicks in on the dirty page, lock the page and every > > thing as usual , sees it is GUPed and inform the block layer to > > use a bounce page > > No. The solution John, Dan & I have been looking at is to take the > dirty page off the LRU while it is pinned by GUP. It will never be > found for writeback. > > That's not the end of the story though. Other parts of the kernel (eg > msync) also need to be taught to stay away from pages which are pinned > by GUP. But the idea is that no page gets written back to storage while > it's pinned by GUP. Only when the last GUP ends is the page returned > to the list of dirty pages. > > > - block layer copy the page to a bounce page effectively creating > > a snapshot of what is the content of the real page. This allows > > everything in block layer that need stable content to work on > > the bounce page (raid, stripping, encryption, ...) > > - once write back is done the page is not marked clean but stays > > dirty, this effectively disable things like COW for filesystem > > and other feature that expect page_mkwrite between write back. > > AFAIK it is believe that it is something acceptable > > So none of this is necessary. With the solution you are proposing we loose GUP fast and we have to allocate a structure for each page that is under GUP, and the LRU changes too. Moreover by not writing back there is a greater chance of data loss. I will do patches with the mapcount solution that require very little change and people will be able to choose which solution they prefer. Personaly i prefer the mapcount solution as it is less invasive. Cheers, Jérôme