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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 60D0CC2D0F4 for ; Wed, 8 Apr 2020 22:26:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 388FA20753 for ; Wed, 8 Apr 2020 22:26:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726552AbgDHW0j (ORCPT ); Wed, 8 Apr 2020 18:26:39 -0400 Received: from mga07.intel.com ([134.134.136.100]:54798 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726469AbgDHW0j (ORCPT ); Wed, 8 Apr 2020 18:26:39 -0400 IronPort-SDR: 6pq+mChRpqA0WZFWT5H3wsSm4AAYwnhYuM2DQYwha+bofPAcWqj3NHJUCLRkY3+/zvz4g5nL99 1FZFTvi+Qe5Q== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga004.fm.intel.com ([10.253.24.48]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Apr 2020 15:26:37 -0700 IronPort-SDR: S7w5tzhSIUMK1s0Evt+AKVNgzUL9c21YMfReFPfYHTUsV68lgWkEkFlNelu5tYpfqWc5BiUCib ekmJgdujvqMw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,360,1580803200"; d="scan'208";a="275594942" Received: from iweiny-desk2.sc.intel.com ([10.3.52.147]) by fmsmga004.fm.intel.com with ESMTP; 08 Apr 2020 15:26:37 -0700 Date: Wed, 8 Apr 2020 15:26:36 -0700 From: Ira Weiny To: Dave Chinner Cc: Jan Kara , linux-kernel@vger.kernel.org, "Darrick J. Wong" , Dan Williams , Christoph Hellwig , "Theodore Y. Ts'o" , Jeff Moyer , linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH V6 7/8] fs/xfs: Change xfs_ioctl_setattr_dax_invalidate() to xfs_ioctl_dax_check() Message-ID: <20200408222636.GC664132@iweiny-DESK2.sc.intel.com> References: <20200407182958.568475-1-ira.weiny@intel.com> <20200407182958.568475-8-ira.weiny@intel.com> <20200408022318.GJ24067@dread.disaster.area> <20200408095803.GB30172@quack2.suse.cz> <20200408210950.GL24067@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200408210950.GL24067@dread.disaster.area> User-Agent: Mutt/1.11.1 (2018-12-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 09, 2020 at 07:09:50AM +1000, Dave Chinner wrote: > On Wed, Apr 08, 2020 at 11:58:03AM +0200, Jan Kara wrote: > > On Wed 08-04-20 12:23:18, Dave Chinner wrote: > > > On Tue, Apr 07, 2020 at 11:29:57AM -0700, ira.weiny@intel.com wrote: > > > > From: Ira Weiny > > > > > > > > We only support changing FS_XFLAG_DAX on directories. Files get their > > > > flag from the parent directory on creation only. So no data > > > > invalidation needs to happen. > > > > > > Which leads me to ask: how are users and/or admins supposed to > > > remove the flag from regular files once it is set in the filesystem? > > > > > > Only being able to override the flag via the "dax=never" mount > > > option means that once the flag is set, nobody can ever remove it > > > and they can only globally turn off dax if it gets set incorrectly. > > > It also means a global interrupt because all apps on the filesystem > > > need to be stopped so the filesystem can be unmounted and mounted > > > again with dax=never. This is highly unfriendly to admins and users. > > > > > > IOWs, we _must_ be able to clear this inode flag on regular inodes > > > in some way. I don't care if it doesn't change the current in-memory > > > state, but we must be able to clear the flags so that the next time > > > the inodes are instantiated DAX is not enabled for those files... > > > > Well, there's one way to clear the flag: delete the file. If you still care > > about the data, you can copy the data first. It isn't very convenient, I > > agree, and effectively means restarting whatever application that is using > > the file. > > Restarting the application is fine. Having to backup/restore or copy > the entire data set just to turn off an inode flag? That's not a > viable management strategy. We could be talking about terabytes of > data here. > > I explained how we can safely remove the flag in the other branch of > this thread... > > > But it seems like more understandable API than letting user clear > > the on-disk flag but the inode will still use DAX until kernel decides to > > evict the inode > > Certainly doesn't seem that way to me. "stop app, clear flags, drop > caches, restart app" is a pretty simple, easy thing to do for an > admin. I want to be clear here: I think this is reasonable. However, I don't see consensus for that interface. Christoph in particular said that a 'lazy change' is: "... straight from the playbook for arcane and confusing API designs." "But returning an error and doing a lazy change anyway is straight from the playbook for arcane and confusing API designs." -- https://lore.kernel.org/lkml/20200403072731.GA24176@lst.de/ Did I somehow misunderstand this? Again for this patch set, 5.8, lets leave that alone for now. I think if we disable setting this on files right now we can still allow it in the future as another step forward. > > Especially compared to process that is effectively "stop app, backup > data set, delete data set, clear flags, restore data set, restart > app" > > > - because that often means you need to restart the > > application using the file anyway for the flag change to have any effect. > > That's a trivial requirement compared to the downtime and resource > cost of a data set backup/restore just to clear inode flags.... > I agree but others do not. This still provides a baby step forward and some granularity for those who plan out the creation of their files. Ira