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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 D7667C3A589 for ; Thu, 15 Aug 2019 13:26:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B75CC2133F for ; Thu, 15 Aug 2019 13:26:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732267AbfHON0Z (ORCPT ); Thu, 15 Aug 2019 09:26:25 -0400 Received: from mx2.suse.de ([195.135.220.15]:45032 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731846AbfHON0Z (ORCPT ); Thu, 15 Aug 2019 09:26:25 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id C4A59AE12; Thu, 15 Aug 2019 13:26:22 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 1C66F1E4200; Thu, 15 Aug 2019 15:26:22 +0200 (CEST) Date: Thu, 15 Aug 2019 15:26:22 +0200 From: Jan Kara To: John Hubbard Cc: Ira Weiny , Andrew Morton , Christoph Hellwig , Dan Williams , Dave Chinner , Jan Kara , Jason Gunthorpe , =?iso-8859-1?B?Suly9G1l?= Glisse , LKML , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-rdma@vger.kernel.org Subject: Re: [RFC PATCH 2/2] mm/gup: introduce vaddr_pin_pages_remote() Message-ID: <20190815132622.GG14313@quack2.suse.cz> References: <20190812015044.26176-1-jhubbard@nvidia.com> <20190812015044.26176-3-jhubbard@nvidia.com> <20190812234950.GA6455@iweiny-DESK2.sc.intel.com> <38d2ff2f-4a69-e8bd-8f7c-41f1dbd80fae@nvidia.com> <20190813210857.GB12695@iweiny-DESK2.sc.intel.com> <90e5cd11-fb34-6913-351b-a5cc6e24d85d@nvidia.com> <20190814234959.GA463@iweiny-DESK2.sc.intel.com> <2cbdf599-2226-99ae-b4d5-8909a0a1eadf@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed 14-08-19 20:01:07, John Hubbard wrote: > On 8/14/19 5:02 PM, John Hubbard wrote: > > On 8/14/19 4:50 PM, Ira Weiny wrote: > > > On Tue, Aug 13, 2019 at 05:56:31PM -0700, John Hubbard wrote: > > > > On 8/13/19 5:51 PM, John Hubbard wrote: > > > > > On 8/13/19 2:08 PM, Ira Weiny wrote: > > > > > > On Mon, Aug 12, 2019 at 05:07:32PM -0700, John Hubbard wrote: > > > > > > > On 8/12/19 4:49 PM, Ira Weiny wrote: > > > > > > > > On Sun, Aug 11, 2019 at 06:50:44PM -0700, john.hubbard@gmail.com wrote: > > > > > > > > > From: John Hubbard > > > > > > > ... > > > > > > Finally, I struggle with converting everyone to a new call.  It is more > > > > > > overhead to use vaddr_pin in the call above because now the GUP code is going > > > > > > to associate a file pin object with that file when in ODP we don't need that > > > > > > because the pages can move around. > > > > > > > > > > What if the pages in ODP are file-backed? > > > > > > > > > > > > > oops, strike that, you're right: in that case, even the file system case is covered. > > > > Don't mind me. :) > > > > > > Ok so are we agreed we will drop the patch to the ODP code?  I'm going to keep > > > the FOLL_PIN flag and addition in the vaddr_pin_pages. > > > > > > > Yes. I hope I'm not overlooking anything, but it all seems to make sense to > > let ODP just rely on the MMU notifiers. > > > > Hold on, I *was* forgetting something: this was a two part thing, and > you're conflating the two points, but they need to remain separate and > distinct. There were: > > 1. FOLL_PIN is necessary because the caller is clearly in the use case that > requires it--however briefly they might be there. As Jan described it, > > "Anything that gets page reference and then touches page data (e.g. > direct IO) needs the new kind of tracking so that filesystem knows > someone is messing with the page data." [1] So when the GUP user uses MMU notifiers to stop writing to pages whenever they are writeprotected with page_mkclean(), they don't really need page pin - their access is then fully equivalent to any other mmap userspace access and filesystem knows how to deal with those. I forgot out this case when I wrote the above sentence. So to sum up there are three cases: 1) DIO case - GUP references to pages serving as DIO buffers are needed for relatively short time, no special synchronization with page_mkclean() or munmap() => needs FOLL_PIN 2) RDMA case - GUP references to pages serving as DMA buffers needed for a long time, no special synchronization with page_mkclean() or munmap() => needs FOLL_PIN | FOLL_LONGTERM This case has also a special case when the pages are actually DAX. Then the caller additionally needs file lease and additional file_pin structure is used for tracking this usage. 3) ODP case - GUP references to pages serving as DMA buffers, MMU notifiers used to synchronize with page_mkclean() and munmap() => normal page references are fine. Honza -- Jan Kara SUSE Labs, CR