linux-kernel - Re: [PATCH v3 00/12] Enable per-file/directory DAX operations V3

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Date:   Thu, 13 Feb 2020 16:16:17 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Jeff Moyer <jmoyer@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 00/12] Enable per-file/directory DAX operations V3

On Thu, Feb 13, 2020 at 3:29 PM Ira Weiny <ira.weiny@...el.com> wrote:
>
> On Thu, Feb 13, 2020 at 11:58:39AM -0800, Darrick J. Wong wrote:
> > On Thu, Feb 13, 2020 at 11:05:13AM -0800, Ira Weiny wrote:
> > > On Thu, Feb 13, 2020 at 11:01:57AM -0800, 'Ira Weiny' wrote:
> > > > On Wed, Feb 12, 2020 at 02:49:48PM -0500, Jeff Moyer wrote:
> > > > > Ira Weiny <ira.weiny@...el.com> writes:
> > > > >
> > >
> > > [snip]
> > >
> > > > > Given that we document the dax mount
> > > > > option as "the way to get dax," it may be a good idea to allow for a
> > > > > user to selectively disable dax, even when -o dax is specified.  Is that
> > > > > possible?
> > > >
> > > > Not with this patch set.  And I'm not sure how that would work.  The idea was
> > > > that -o dax was simply an override for users who were used to having their
> > > > entire FS be dax.  We wanted to depreciate the use of "-o dax" in general.  The
> > > > individual settings are saved so I don't think it makes sense to ignore the -o
> > > > dax in favor of those settings.  Basically that would IMO make the -o dax
> > > > useless.
> > >
> > > Oh and I forgot to mention that setting 'dax' on the root of the FS basically
> > > provides '-o dax' functionality by default with the ability to "turn it off"
> > > for files.
> >
> > Please don't further confuse FS_XFLAG_DAX and S_DAX.
>
> Yes...  the above text is wrong WRT statx.  But setting the physical
> XFS_DIFLAG2_DAX flag on the root directory will by default cause all files and
> directories created there to be XFS_DIFLAG2_DAX and so forth on down the tree
> unless explicitly changed.  This will be the same as mounting with '-o dax' but
> with the ability to turn off dax for individual files.  Which I think is the
> functionality Jeff is wanting.

To be clear you mean turn off XFS_DIFLAG2_DAX, not mask S_DAX when you
say "turn off dax", right?

The mount option simply forces "S_DAX" on all regular files as long as
the underlying device (or soon to be superblock for virtiofs) supports
it. There is no method to mask S_DAX when the filesystem was mounted
with -o dax. Otherwise we would seem to need yet another physical flag
to "always disable" dax.