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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 CD2B6C169C4 for ; Wed, 6 Feb 2019 18:35:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9B755218D3 for ; Wed, 6 Feb 2019 18:35:10 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="vANjs2HZ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726892AbfBFSfI (ORCPT ); Wed, 6 Feb 2019 13:35:08 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:37312 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726022AbfBFSfI (ORCPT ); Wed, 6 Feb 2019 13:35:08 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=qI29K96IrArheGtskdty/M4YlvDQ8BSIpYtPNkZIORY=; b=vANjs2HZBeSGsIJ3lVWi8DZVD FXcQzXRiaXnlbJbaNHMMz5rB1uCRmtDS05jhtlnMlprwaSI1fr/BZV6C57RLXpkYFPAONlFd1M7E+ uC2CHtoQOb7aU3uYybJzUyhpS+S7KR9tW27Zg6wJBc1hGIOKddlyTEvjBIPvAtSYc1IJWPJVr3d/c 5PeTnFCETxFmuk7KuhJTmmutnLRhKJtW+eIx7FpDZ/Ouh2Q0O2ML8CT5BmKJx5v4d5ETgFpkmJrzm ouOa+KlxEKtj5CBvL0Tc9uyen/ih/etkhU8eIZfoB15Hol1cNZhC89ThfrkdyVwGSF5bXT8Zgwyw4 BAR3PItPw==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1grS2K-0005xE-8p; Wed, 06 Feb 2019 18:35:04 +0000 Date: Wed, 6 Feb 2019 10:35:04 -0800 From: Matthew Wilcox To: Doug Ledford Cc: Jason Gunthorpe , Jan Kara , Ira Weiny , lsf-pc@lists.linux-foundation.org, linux-rdma@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, John Hubbard , Jerome Glisse , Dan Williams , Dave Chinner , Michal Hocko Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA Message-ID: <20190206183503.GO21860@bombadil.infradead.org> References: <20190205175059.GB21617@iweiny-DESK2.sc.intel.com> <20190206095000.GA12006@quack2.suse.cz> <20190206173114.GB12227@ziepe.ca> <20190206175233.GN21860@bombadil.infradead.org> <47820c4d696aee41225854071ec73373a273fd4a.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <47820c4d696aee41225854071ec73373a273fd4a.camel@redhat.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 06, 2019 at 01:32:04PM -0500, Doug Ledford wrote: > On Wed, 2019-02-06 at 09:52 -0800, Matthew Wilcox wrote: > > On Wed, Feb 06, 2019 at 10:31:14AM -0700, Jason Gunthorpe wrote: > > > On Wed, Feb 06, 2019 at 10:50:00AM +0100, Jan Kara wrote: > > > > > > > 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. > > > > > > This takes a trip through userspace since the communication protocol > > > is entirely managed in userspace. > > > > > > Most existing communication protocols don't have a 'cancel operation'. > > > > > > > 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. > > > > > > We can always rip things away from the userspace.. However.. > > > > > > > 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. > > > > > > I think the general observation is that people who want to do DAX & > > > RDMA want it to actually work, without data corruption, random process > > > kills or random communication failures. > > > > > > Really, few users would actually want to run in a system where revoke > > > can be triggered. > > > > > > So.. how can the FS/MM side provide a guarantee to the user that > > > revoke won't happen under a certain system design? > > > > Most of the cases we want revoke for are things like truncate(). > > Shouldn't happen with a sane system, but we're trying to avoid users > > doing awful things like being able to DMA to pages that are now part of > > a different file. > > Why is the solution revoke then? Is there something besides truncate > that we have to worry about? I ask because EBUSY is not currently > listed as a return value of truncate, so extending the API to include > EBUSY to mean "this file has pinned pages that can not be freed" is not > (or should not be) totally out of the question. > > Admittedly, I'm coming in late to this conversation, but did I miss the > portion where that alternative was ruled out? That's my preferred option too, but the preponderance of opinion leans towards "We can't give people a way to make files un-truncatable".