[Virtio-fs] [PATCH v6 2/7] fuse: make DAX mount option a tri-state

[Ira Weiny]

On Fri, Oct 22, 2021 at 02:54:03PM +0800, JeffleXu wrote:
> cc [Ira Weiny], author of per inode DAX on xfs/ext4
> 
> On 10/20/21 11:17 PM, Vivek Goyal wrote:
> > On Wed, Oct 20, 2021 at 10:52:38AM +0800, JeffleXu wrote:
> >>
> >>
> >> On 10/18/21 10:10 PM, Vivek Goyal wrote:
> >>> On Mon, Oct 11, 2021 at 11:00:47AM +0800, Jeffle Xu wrote:
> >>>> We add 'always', 'never', and 'inode' (default). '-o dax' continues to
> >>>> operate the same which is equivalent to 'always'. To be consistemt with
> >>>> ext4/xfs's tri-state mount option, when neither '-o dax' nor '-o dax='
> >>>> option is specified, the default behaviour is equal to 'inode'.
> >>>
> >>> Hi Jeffle,
> >>>
> >>> I am not sure when  -o "dax=inode"  is used as a default? If user
> >>> specifies, "-o dax" then it is equal to "-o dax=always", otherwise
> >>> user will explicitly specify "-o dax=always/never/inode". So when
> >>> is dax=inode is used as default?
> >>
> >> That means when neither '-o dax' nor '-o dax=always/never/inode' is
> >> specified, it is actually equal to '-o dax=inode', which is also how
> >> per-file DAX on ext4/xfs works.
> > 

It's been a while so I'm fuzzy on the details of the discussions but yes that
is the way things are now in the code.

> > [ CC dave chinner] 
> > 
> > Is it not change of default behavior for ext4/xfs as well. My
> > understanding is that prior to this new dax options, "-o dax" enabled
> > dax on filesystem and if user did not specify it, DAX is disbaled
> > by default.

Technically it does change default behavior...  However, NOT in a way which
breaks anything.  See below.

> > 
> > Now after introduction of "-o dax=always/never/inode", if suddenly
> > "-o dax=inode" became the default if user did not specify anything,
> > that's change of behavior.

Technically yes but not in a broken way.

> >
> > Is that intentional. If given a choice,
> > I would rather not change default and ask user to opt-in for
> > appropriate dax functionality.

There is no need for this.

> > 
> > Dave, you might have thoughts on this. It makes me uncomfortable to
> > change virtiofs dax default now just because other filesytems did it.
> > 
> 
> I can only find the following discussions about the earliest record on
> this tri-state mount option:
> 
> https://lore.kernel.org/lkml/20200316095509.GA13788@lst.de/
> https://lore.kernel.org/lkml/20200401040021.GC56958@magnolia/
> 
> 
> Hi, Ira Weiny,
> 
> Do you have any thought on this, i.e. why the default behavior has
> changed after introduction of per inode dax?

While this is 'technically' different behavior the end user does not see any
difference in behavior if they continue without software changes.  Specifically
specifying nothing continues to operate with all the files on the FS to be
'_not_ DAX'.  While specifying '-o dax' forces DAX on all files.

This expands the default behavior in a backwards compatible manner.  The user
can now enable DAX on some files.  But this is an opt-in on the part of the
user of the FS and again does not change with existing software/scripts/etc.

Does that make sense?

Ira