From patchwork Tue Apr 28 22:19:42 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ira Weiny X-Patchwork-Id: 11515761 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 0A07515AB for ; Tue, 28 Apr 2020 22:19:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EC28820787 for ; Tue, 28 Apr 2020 22:19:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726580AbgD1WTs (ORCPT ); Tue, 28 Apr 2020 18:19:48 -0400 Received: from mga06.intel.com ([134.134.136.31]:32655 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726500AbgD1WTs (ORCPT ); Tue, 28 Apr 2020 18:19:48 -0400 IronPort-SDR: ZDQxUzUBUfQ6GM5vJm+1jsawEtjmVZUDL3FTNfwnlsMEkQkZR/yVdW2cbERcCeMCGvCOc85pYp c9tvP60OW0yA== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Apr 2020 15:19:45 -0700 IronPort-SDR: eAU+Cj8p1QJrbdl6uNR4a/GW6kPJnSrPWsugqOqk5Z+Tp1L/GWzPUu/hvrpUeTkI0AMq5q6TXO zsYEmUALHorg== X-IronPort-AV: E=Sophos;i="5.73,328,1583222400"; d="scan'208";a="257763108" Received: from iweiny-desk2.sc.intel.com (HELO localhost) ([10.3.52.147]) by orsmga003-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 28 Apr 2020 15:19:45 -0700 From: ira.weiny@intel.com To: linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, "Darrick J. Wong" Cc: Ira Weiny , 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, linux-api@vger.kernel.org Subject: [PATCH V11.1] Documentation/dax: Update Usage section Date: Tue, 28 Apr 2020 15:19:42 -0700 Message-Id: <20200428221942.409890-1-ira.weiny@intel.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <'20200428002142.404144-5-ira.weiny@intel.com'> References: <'20200428002142.404144-5-ira.weiny@intel.com'> MIME-Version: 1.0 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org From: Ira Weiny Update the Usage section to reflect the new individual dax selection functionality. Signed-off-by: Ira Weiny --- Changes from V11: Minor changes from Darrick Changes from V10: Clarifications from Dave Add '-c' to xfs_io examples Changes from V9: Fix missing ')' Fix trialing '"' 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 | 142 +++++++++++++++++++++++++++++- 1 file changed, 139 insertions(+), 3 deletions(-) diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt index 679729442fd2..dc1c1aa36cc2 100644 --- a/Documentation/filesystems/dax.txt +++ b/Documentation/filesystems/dax.txt @@ -17,11 +17,147 @@ 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 a persistent flag FS_XFLAG_DAX that can be applied to regular + files and directories. This advisory flag can be set or cleared at any + time, but doing so does not immediately affect the S_DAX state. + + 3. If the persistent FS_XFLAG_DAX flag is set on a directory, this flag will + be inherited by all regular files and subdirectories that are subsequently + created in this directory. Files and subdirectories that exist at the time + this flag is set or cleared on the parent directory are not modified by + this modification of the parent directory. + + 4. There exists dax mount options which can override FS_XFLAG_DAX in the + setting of the S_DAX flag. Given underlying storage which supports DAX the + following hold: + + "-o dax=inode" means "follow FS_XFLAG_DAX" and is the default. + + "-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" is a legacy option which is an alias for "dax=always". + This may be removed in the future so "-o dax=always" is + the preferred method for specifying this behavior. + + NOTE: Modifications to and the inheritance behavior of FS_XFLAG_DAX remain + the same even when the file system is mounted with a dax option. However, + in-core inode state (S_DAX) will be overridden until the file system is + remounted with dax=inode and the inode is evicted from kernel memory. + + 5. The S_DAX policy can be changed via: + + a) Setting the parent directory FS_XFLAG_DAX as needed before files are + created + + b) Setting the appropriate dax="foo" mount option + + c) Changing the FS_XFLAG_DAX on existing regular files and directories. + This has runtime constraints and limitations that are described in 6) + below. + + 6. When changing the S_DAX policy via toggling the persistent FS_XFLAG_DAX flag, + the change in behaviour for existing regular files may not occur + immediately. If the change must take effect immediately, the administrator + needs to: + + a) stop the application so there are no active references to the data set + the policy change will affect + + b) evict the data set from kernel caches so it will be re-instantiated when + the application is restarted. This can be achieved by: + + i. drop-caches + ii. a filesystem unmount and mount cycle + iii. a system reboot + + +Details +------- + +There are 2 per-file dax flags. One is a persistent inode setting (FS_XFLAG_DAX) +and the other is a volatile flag indicating the active state of the feature +(S_DAX). + +FS_XFLAG_DAX is preserved within the file system. This persistent config +setting can be set, cleared and/or queried using the FS_IOC_FS[GS]ETXATTR ioctl +(see ioctl_xfs_fsgetxattr(2)) or an utility such as 'xfs_io'. + +New 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. + +To clarify inheritance, here are 3 examples: + +Example A: + +mkdir -p a/b/c +xfs_io -c '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 -c '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 -c '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 instantiated in +memory by the kernel. It is set based on the underlying media support, the +value of FS_XFLAG_DAX and the file system's dax mount option. + +statx can be used to query S_DAX. NOTE that only regular files will ever have +S_DAX set and therefore statx will never indicate that S_DAX is set on +directories. + +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. + Implementation Tips for Block Driver Writers