linux-kernel - Re: [PATCH V7 9/9] Documentation/dax: Update Usage section

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Date:   Tue, 14 Apr 2020 12:48:48 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V7 9/9] Documentation/dax: Update Usage section

On Mon, Apr 13, 2020 at 10:12:22PM -0700, Dan Williams wrote:
> On Mon, Apr 13, 2020 at 9:38 PM Ira Weiny <ira.weiny@...el.com> wrote:
> >
> > On Mon, Apr 13, 2020 at 09:19:12AM -0700, Darrick J. Wong wrote:
> > > On Sun, Apr 12, 2020 at 10:40:46PM -0700, ira.weiny@...el.com wrote:
> > > > From: Ira Weiny <ira.weiny@...el.com>
> > > >
> > > > Update the Usage section to reflect the new individual dax selection
> > > > functionality.
> > >
> > > Yum. :)
> > >
> > > > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > > >
> > > > ---
> > > > 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 | 166 +++++++++++++++++++++++++++++-
> > > >  1 file changed, 163 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
> > > > index 679729442fd2..af14c1b330a9 100644
> > > > --- a/Documentation/filesystems/dax.txt
> > > > +++ b/Documentation/filesystems/dax.txt
> > > > @@ -17,11 +17,171 @@ 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 2 filesystems support DAX, ext4 and xfs.  Enabling DAX on them is
> > > > +different at this time.
> > >
> > > I thought ext2 supports DAX?
> >
> > Not that I know of?  Does it?
> 
> Yes. Seemed like a good idea at the time, but in retrospect...

Ah ok...   Is there an objection to leaving ext2 as a global mount option?
Updating the doc is easy enough.

Ira

> 
> In fairness I believe this was also an olive branch to XIP users that
> were transitioned to DAX, so they did not also need to transition
> filesystems.