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,URIBL_BLOCKED,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 3C33AC169C4 for ; Wed, 6 Feb 2019 09:50:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 01F6920B1F for ; Wed, 6 Feb 2019 09:50:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728393AbfBFJuE (ORCPT ); Wed, 6 Feb 2019 04:50:04 -0500 Received: from mx2.suse.de ([195.135.220.15]:42260 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725965AbfBFJuD (ORCPT ); Wed, 6 Feb 2019 04:50:03 -0500 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 263D1AE43; Wed, 6 Feb 2019 09:50:02 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 7E0631E3E15; Wed, 6 Feb 2019 10:50:00 +0100 (CET) Date: Wed, 6 Feb 2019 10:50:00 +0100 From: Jan Kara To: Ira Weiny Cc: lsf-pc@lists.linux-foundation.org, linux-rdma@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, John Hubbard , Jan Kara , Jerome Glisse , Dan Williams , Matthew Wilcox , Jason Gunthorpe , Dave Chinner , Doug Ledford , Michal Hocko Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA Message-ID: <20190206095000.GA12006@quack2.suse.cz> References: <20190205175059.GB21617@iweiny-DESK2.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190205175059.GB21617@iweiny-DESK2.sc.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 05-02-19 09:50:59, Ira Weiny wrote: > The problem: Once we have pages marked as GUP-pinned how should various > subsystems work with those markings. > > The current work for John Hubbards proposed solutions (part 1 and 2) is > progressing.[1] But the final part (3) of his solution is also going to take > some work. > > In Johns presentation he lists 3 alternatives for gup-pinned pages: > > 1) Hold off try_to_unmap > 2) Allow writeback while pinned (via bounce buffers) > [Note this will not work for DAX] Well, but DAX does not need it because by definition there's nothing to writeback :) > 3) Use a "revocable reservation" (or lease) on those pages > 4) Pin the blocks as busy in the FS allocator > > The problem with lease's on pages used by RDMA is that the references to > these pages is not local to the machine. Once the user has been given > access to the page they, through the use of a remote tokens, give a > reference to that page to remote nodes. This is the core essence of > RDMA, and like it or not, something which is increasingly used by major > Linux users. > > Therefore we need to discuss the extent by which leases are appropriate and > what happens should a lease be revoked which a user does not respond to. I don't know the RDMA hardware so this is just an opinion of filesystem / mm guy but my idea how this should work would be: MM/FS asks for lease to be revoked. The revoke handler agrees with the other side on cancelling RDMA or whatever and drops the page pins. Now I understand there can be HW / communication failures etc. in which case the driver could either block waiting or make sure future IO will fail and drop the pins. But under normal conditions there should be a way to revoke the access. And if the HW/driver cannot support this, then don't let it anywhere near DAX filesystem. Honza -- Jan Kara SUSE Labs, CR