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 C6B98C2BB1D for ; Tue, 14 Apr 2020 20:00:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A8E7720656 for ; Tue, 14 Apr 2020 20:00:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2505382AbgDNUAq (ORCPT ); Tue, 14 Apr 2020 16:00:46 -0400 Received: from mga14.intel.com ([192.55.52.115]:10312 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2505359AbgDNUAV (ORCPT ); Tue, 14 Apr 2020 16:00:21 -0400 IronPort-SDR: Adh2bSGjpAj5C5lb+kuGh7dFOpnidmodv17mXdXnZ8tvgWLcRe0IoTJPbR3XdIFvKyY4MNgZz4 7ZwPp4l1EWpg== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga007.jf.intel.com ([10.7.209.58]) by fmsmga103.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 14 Apr 2020 13:00:17 -0700 IronPort-SDR: lhbGUOmvotbhMVvY0OCmgMZTQi0sG5VUHFqwvb87jAD3F8K/dEr9Zz6Qqi7g2um/4m6S5r9ckW Ni0dXCw+E5ew== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,384,1580803200"; d="scan'208";a="242092945" Received: from iweiny-desk2.sc.intel.com ([10.3.52.147]) by orsmga007.jf.intel.com with ESMTP; 14 Apr 2020 13:00:15 -0700 Date: Tue, 14 Apr 2020 13:00:15 -0700 From: Ira Weiny To: "Darrick J. Wong" Cc: Dan Williams , Linux Kernel Mailing List , Dave Chinner , Christoph Hellwig , "Theodore Y. Ts'o" , Jan Kara , Jeff Moyer , linux-ext4 , linux-xfs , linux-fsdevel Subject: Re: [PATCH V7 9/9] Documentation/dax: Update Usage section Message-ID: <20200414200015.GF1853609@iweiny-DESK2.sc.intel.com> References: <20200413054046.1560106-1-ira.weiny@intel.com> <20200413054046.1560106-10-ira.weiny@intel.com> <20200414161509.GF6742@magnolia> <20200414195754.GH6742@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200414195754.GH6742@magnolia> 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 Tue, Apr 14, 2020 at 12:57:54PM -0700, Darrick J. Wong wrote: > On Tue, Apr 14, 2020 at 12:04:57PM -0700, Dan Williams wrote: > > On Tue, Apr 14, 2020 at 9:15 AM Darrick J. Wong wrote: [snip] > > > > > + > > > > > +Enabling DAX on xfs > > > > > +------------------- > > > > > + > > > > > +Summary > > > > > +------- > > > > > + > > > > > + 1. There exists an in-kernel access mode flag S_DAX that is set when > > > > > + file accesses go directly to persistent memory, bypassing the page > > > > > + cache. > > > > > > > > I had reserved some quibbling with this wording, but now that this is > > > > being proposed as documentation I'll let my quibbling fly. "dax" may > > > > imply, but does not require persistent memory nor does it necessarily > > > > "bypass page cache". For example on configurations that support dax, > > > > but turn off MAP_SYNC (like virtio-pmem), a software flush is > > > > required. Instead, if we're going to define "dax" here I'd prefer it > > > > be a #include of the man page definition that is careful (IIRC) to > > > > only talk about semantics and not backend implementation details. In > > > > other words, dax is to page-cache as direct-io is to page cache, > > > > effectively not there, but dig a bit deeper and you may find it. > > > > > > Uh, which manpage? Are you talking about the MAP_SYNC documentation? > > > > No, I was referring to the proposed wording for STATX_ATTR_DAX. > > There's no reason for this description to say anything divergent from > > that description. > > Ahh, ok. Something like this, then: > > 1. There exists an in-kernel access mode flag S_DAX. When set, the > file is in the DAX (cpu direct access) state. DAX state attempts to > minimize software cache effects for both I/O and memory mappings of > this file. The S_DAX state is exposed to userspace via the > STATX_ATTR_DAX statx flag. > > See the STATX_ATTR_DAX in the statx(2) manpage for more information. We crossed in the ether!!! I propose even less details here... Leave all the details to the man page. 1. There exists an in-kernel access mode flag S_DAX that is set when file accesses is enabled for 'DAX'. Applications must call statx to discover the current S_DAX state (STATX_ATTR_DAX). See the man page for statx for more details. Ira