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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 47E13C55189 for ; Wed, 22 Apr 2020 09:47:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2B2A62071E for ; Wed, 22 Apr 2020 09:47:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726183AbgDVJrZ (ORCPT ); Wed, 22 Apr 2020 05:47:25 -0400 Received: from mx2.suse.de ([195.135.220.15]:33782 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725994AbgDVJrZ (ORCPT ); Wed, 22 Apr 2020 05:47:25 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 430DAABCC; Wed, 22 Apr 2020 09:47:22 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id D54451E0B00; Wed, 22 Apr 2020 11:47:21 +0200 (CEST) Date: Wed, 22 Apr 2020 11:47:21 +0200 From: Jan Kara To: ira.weiny@intel.com Cc: linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, "Darrick J. Wong" , Al Viro , Jan Kara , Dan Williams , Dave Chinner , Christoph Hellwig , "Theodore Y. Ts'o" , Jeff Moyer , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH V9 11/11] Documentation/dax: Update Usage section Message-ID: <20200422094721.GG8775@quack2.suse.cz> References: <20200421191754.3372370-1-ira.weiny@intel.com> <20200421191754.3372370-12-ira.weiny@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200421191754.3372370-12-ira.weiny@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 21-04-20 12:17:53, ira.weiny@intel.com wrote: > From: Ira Weiny > > Update the Usage section to reflect the new individual dax selection > functionality. > > Signed-off-by: Ira Weiny Looks good to me. You can add: Reviewed-by: Jan Kara Honza > > --- > Changes from V8: > Updates from Darrick > > Changes from V7: > Cleanups/clarifications from Darrick and Dan > > Changes from V6: > Update to allow setting FS_XFLAG_DAX any time. > Update with list of behaviors from Darrick > https://lore.kernel.org/lkml/20200409165927.GD6741@magnolia/ > > Changes from V5: > Update to reflect the agreed upon semantics > https://lore.kernel.org/lkml/20200405061945.GA94792@iweiny-DESK2.sc.intel.com/ > --- > Documentation/filesystems/dax.txt | 164 +++++++++++++++++++++++++++++- > 1 file changed, 161 insertions(+), 3 deletions(-) > > diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt > index 679729442fd2..8f4ab08be715 100644 > --- a/Documentation/filesystems/dax.txt > +++ b/Documentation/filesystems/dax.txt > @@ -17,11 +17,169 @@ For file mappings, the storage device is mapped directly into userspace. > Usage > ----- > > -If you have a block device which supports DAX, you can make a filesystem > +If you have a block device which supports DAX, you can make a file system > on it as usual. The DAX code currently only supports files with a block > size equal to your kernel's PAGE_SIZE, so you may need to specify a block > -size when creating the filesystem. When mounting it, use the "-o dax" > -option on the command line or add 'dax' to the options in /etc/fstab. > +size when creating the file system. > + > +Currently 3 filesystems support DAX: ext2, ext4 and xfs. Enabling DAX on them > +is different. > + > +Enabling DAX on ext4 and ext2 > +----------------------------- > + > +When mounting the filesystem, use the "-o dax" option on the command line or > +add 'dax' to the options in /etc/fstab. This works to enable DAX on all files > +within the filesystem. It is equivalent to the '-o dax=always' behavior below. > + > + > +Enabling DAX on xfs > +------------------- > + > +Summary > +------- > + > + 1. There exists an in-kernel file access mode flag S_DAX that corresponds to > + the statx flag STATX_ATTR_DAX. See the manpage for statx(2) for details > + about this access mode. > + > + 2. There exists an advisory file inode flag FS_XFLAG_DAX that is > + inherited from the parent directory FS_XFLAG_DAX inode flag at file > + creation time. This advisory flag can be set or cleared at any > + time, but doing so does not immediately affect the S_DAX state. > + > + Unless overridden by mount options (see (3)), if FS_XFLAG_DAX is set > + and the fs is on pmem then it will enable S_DAX at inode load time; > + if FS_XFLAG_DAX is not set, it will not enable S_DAX. > + > + 3. There exists a dax= mount option. > + > + "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX." > + > + "-o dax=always" means "always set S_DAX (at least on pmem), > + and ignore FS_XFLAG_DAX." > + > + "-o dax" is an alias for "dax=always". > + > + "-o dax=inode" means "follow FS_XFLAG_DAX" and is the default. > + > + 4. There exists an advisory directory inode flag FS_XFLAG_DAX that can > + be set or cleared at any time. The flag state is inherited by any files or > + subdirectories when they are created within that directory. > + > + 5. Programs that require a specific file access mode (DAX or not DAX) > + can do one of the following: > + > + (a) Create files in directories that the FS_XFLAG_DAX flag set as > + needed; or > + > + (b) Have the administrator set an override via mount option; or > + > + (c) Set or clear the file's FS_XFLAG_DAX flag as needed. Programs > + must then cause the kernel to evict the inode from memory. This > + can be done by: > + > + i> Closing the file and re-opening the file and using statx to > + see if the fs has changed the S_DAX flag; and > + > + ii> If the file still does not have the desired S_DAX access > + mode, either unmount and remount the filesystem, or close > + the file and use drop_caches. > + > + 6. It is expected that users who want to squeeze every last bit of performance > + out of the particular rough and tumble bits of their storage will also be > + exposed to the difficulties of what happens when the operating system can't > + totally virtualize those hardware capabilities. DAX is such a feature. > + > + > +Details > +------- > + > +There are 2 per-file dax flags. One is a physical inode setting (FS_XFLAG_DAX) > +and the other a currently enabled state (S_DAX). > + > +FS_XFLAG_DAX is maintained, on disk, on individual inodes. It is preserved > +within the file system. This 'physical' config setting can be set using an > +ioctl and/or an application such as "xfs_io -c 'chattr [-+]x'". Files and > +directories automatically inherit FS_XFLAG_DAX from their parent directory > +_when_ _created_. Therefore, setting FS_XFLAG_DAX at directory creation time > +can be used to set a default behavior for an entire sub-tree. (Doing so on the > +root directory acts to set a default for the entire file system.) > + > +To clarify inheritance here are 3 examples: > + > +Example A: > + > +mkdir -p a/b/c > +xfs_io 'chattr +x' a > +mkdir a/b/c/d > +mkdir a/e > + > + dax: a,e > + no dax: b,c,d > + > +Example B: > + > +mkdir a > +xfs_io 'chattr +x' a > +mkdir -p a/b/c/d > + > + dax: a,b,c,d > + no dax: > + > +Example C: > + > +mkdir -p a/b/c > +xfs_io 'chattr +x' c > +mkdir a/b/c/d > + > + dax: c,d > + no dax: a,b > + > + > +The current enabled state (S_DAX) is set when a file inode is _loaded_ based on > +the underlying media support, the value of FS_XFLAG_DAX, and the file systems > +dax mount option setting. See below. > + > +statx can be used to query S_DAX. NOTE that a directory will never have S_DAX > +set and therefore statx will never indicate that S_DAX is set on directories. > + > +NOTE: Setting the FS_XFLAG_DAX (specifically or through inheritance) occurs > +even if the underlying media does not support dax and/or the file system is > +overridden with a mount option. > + > + > +Overriding FS_XFLAG_DAX (dax= mount option) > +------------------------------------------- > + > +There exists a dax mount option. Using the mount option does not change the > +physical configured state of individual files but overrides the S_DAX operating > +state when inodes are loaded. > + > +Given underlying media support, the dax mount option is a tri-state option > +(never, always, inode) with the following meanings: > + > + "-o dax=never" means "never set S_DAX, ignore FS_XFLAG_DAX" > + "-o dax=always" means "always set S_DAX, ignore FS_XFLAG_DAX" > + "-o dax" by itself means "dax=always" to remain compatible with older > + kernels > + "-o dax=inode" means "follow FS_XFLAG_DAX" > + > +The default state is 'inode'. Given underlying media support, the following > +algorithm is used to determine the effective mode of the file S_DAX on a > +capable device. > + > + S_DAX = FS_XFLAG_DAX; > + > + if (dax_mount == "always") > + S_DAX = true; > + else if (dax_mount == "off" > + S_DAX = false; > + > +To reiterate: Setting, and inheritance, continues to affect FS_XFLAG_DAX even > +while the file system is mounted with a dax override. However, in-core inode > +state (S_DAX) will continue to be overridden until the filesystem is remounted > +with dax=inode and the inode is evicted." > > > Implementation Tips for Block Driver Writers > -- > 2.25.1 > -- Jan Kara SUSE Labs, CR