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

Dan Williams
2020-02-14T00:16:33Z

On Thu, Feb 13, 2020 at 3:29 PM Ira Weiny <ira.weiny@intel.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@intel.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.