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From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Christoph Hellwig <hch@lst.de>,
	Dave Chinner <david@fromorbit.com>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>,
	Ira Weiny <ira.weiny@intel.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5
Date: Wed, 1 Apr 2020 04:00:21 +0000 (UTC)
Message-ID: <20200401040021.GC56958@magnolia> (raw)
In-Reply-To: <20200316095509.GA13788@lst.de>

On Mon, Mar 16, 2020 at 10:55:09AM +0100, Christoph Hellwig wrote:
> On Mon, Mar 16, 2020 at 10:52:24AM +0100, Jan Kara wrote:
> > > This sounds reasonable to me.
> > > 
> > > As for deprecating the mount option, I think at a minimum it needs to
> > > continue be accepted as an option even if it is ignored to not break
> > > existing setups.
> > 
> > Agreed. But that's how we usually deprecate mount options. Also I'd say
> > that statx() support for reporting DAX state and some education of
> > programmers using DAX is required before we deprecate the mount option
> > since currently applications check 'dax' mount option to determine how much
> > memory they need to set aside for page cache before they consume everything
> > else on the machine...
> 
> I don't even think we should deprecate it.  It isn't painful to maintain
> and actually useful for testing.  Instead we should expand it into a
> tristate:
> 
>   dax=off
>   dax=flag
>   dax=always
> 
> where the existing "dax" option maps to "dax=always" and nodax maps
> to "dax=off". and dax=flag becomes the default for DAX capable devices.

That works for me.  In summary:

 - Applications must call statx to discover the current S_DAX state.

 - There exists an advisory file inode flag FS_XFLAG_DAX that can be
   changed on files that have no blocks allocated to them.  Changing
   this flag does not necessarily change the S_DAX state immediately
   but programs can query the S_DAX state via statx.

   If FS_XFLAG_DAX is set and the fs is on pmem then it will always
   enable S_DAX at inode load time; if FS_XFLAG_DAX is not set, it will
   never enable S_DAX.  Unless overridden...

 - There exists a dax= mount option.  dax=off means "never set S_DAX,
   ignore FS_XFLAG_DAX"; dax=always means "always set S_DAX (at least on
   pmem), ignore FS_XFLAG_DAX"; and dax=iflag means "follow FS_XFLAG_DAX"
   and is the default.  "dax" by itself means "dax=always".  "nodax"
   means "dax=off".

 - There exists an advisory directory inode flag FS_XFLAG_DAX that can
   be changed at any time.  The flag state is copied into any files or
   subdirectories created within that directory.  If programs require
   that file access runs in S_DAX mode, they'll have to create those
   files themselves inside a directory with FS_XFLAG_DAX set, or mount
   the fs with dax=always.

Ok?  Let's please get this part finished for 5.8, then we can get back
to arguing about fs-rmap and reflink and dax and whatnot.

--D

  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-27  5:24 ira.weiny
2020-02-27  5:24 ` [PATCH V5 01/12] fs/xfs: Remove unnecessary initialization of i_rwsem ira.weiny
2020-02-27 17:25   ` Ira Weiny
2020-02-27  5:24 ` [PATCH V5 02/12] fs: Remove unneeded IS_DAX() check ira.weiny
2020-02-27  5:24 ` [PATCH V5 03/12] fs/stat: Define DAX statx attribute ira.weiny
2020-02-27  5:24 ` [PATCH V5 04/12] fs/xfs: Isolate the physical DAX flag from enabled ira.weiny
2020-02-27  5:24 ` [PATCH V5 05/12] fs/xfs: Create function xfs_inode_enable_dax() ira.weiny
2020-03-01 22:37   ` Dave Chinner
2020-02-27  5:24 ` [PATCH V5 06/12] fs: Add locking for a dynamic address space operations state ira.weiny
2020-03-02  1:26   ` Dave Chinner
2020-03-02  1:36     ` Dave Chinner
2020-02-27  5:24 ` [PATCH V5 07/12] fs: Prevent DAX state change if file is mmap'ed ira.weiny
2020-02-27  5:24 ` [PATCH V5 08/12] fs/xfs: Hold off aops users while changing DAX state ira.weiny
2020-02-27  5:24 ` [PATCH V5 09/12] fs/xfs: Clean up locking in dax invalidate ira.weiny
2020-02-27  5:24 ` [PATCH V5 10/12] fs/xfs: Allow toggle of effective DAX flag ira.weiny
2020-02-27  5:24 ` [PATCH V5 11/12] fs/xfs: Remove xfs_diflags_to_linux() ira.weiny
2020-02-27  5:24 ` [PATCH V5 12/12] Documentation/dax: Update Usage section ira.weiny
2020-03-05 15:51 ` [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5 Christoph Hellwig
2020-03-09 17:04   ` Ira Weiny
2020-03-11  3:36     ` Darrick J. Wong
2020-03-11  6:29       ` Christoph Hellwig
2020-03-11 17:07         ` Dan Williams
2020-03-16  9:52           ` Jan Kara
2020-03-16  9:55             ` Christoph Hellwig
2020-04-01  4:00               ` Darrick J. Wong [this message]
2020-04-01 10:25                 ` Jan Kara
2020-04-02  8:53                   ` Christoph Hellwig
2020-04-02 20:55                     ` Ira Weiny
2020-04-03  7:27                       ` Christoph Hellwig
2020-04-03 15:48                         ` Ira Weiny
2020-04-03 17:03                           ` Jan Kara
2020-04-03 18:18                             ` Ira Weiny
2020-04-03 18:21                               ` Ira Weiny
2020-04-03 18:37                               ` Darrick J. Wong
2020-04-05  6:19                                 ` Ira Weiny
2020-04-06 10:00                               ` Jan Kara
2020-04-03 18:29                             ` Darrick J. Wong
2020-04-03 16:05                         ` Darrick J. Wong
2020-04-03  4:39                 ` Ira Weiny
2020-03-11  6:39       ` Dave Chinner
2020-03-11  6:44         ` Christoph Hellwig
2020-03-11 17:07           ` Dan Williams
2020-03-12  0:49           ` Dave Chinner
2020-03-12  3:00             ` Darrick J. Wong
2020-03-12  7:27             ` Christoph Hellwig

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