Re: [PATCH V10 00/10] famfs: port into fuse

[Gregory Price]

On Thu, Apr 16, 2026 at 08:56:46AM -0700, Joanne Koong wrote:
> On Tue, Apr 14, 2026 at 5:10 PM John Groves <John@xxxxxxxxxx> wrote:
> >
> > There is a FUSE_DAX_FMAP capability that the kernel may advertise or not
> > at init time; this capability "is" the famfs GET_FMAP AND GET_DAXDEV
> > commands. In the future, if we find a way to use BPF (or some other
> > mechanism) to avoid needing those fuse messages, the kernel could be updated
> > to NEVER advertise the FUSE_DAX_FMAP capability. All of the famfs-specific
> > code could be taken out of kernels that never advertise that capability.
> 
> I’m not sure the capability bit can be used like that (though I am
> hoping it can!). As I understand it, once the kernel advertises a
> capability, it must continue supporting it in future kernels else
> userspace programs that rely on it will break.
> 

FUSE_DAX_FMAP is already conditional on CONFIG_FUSE_DAX, the kernel is
not required to continue advertising FUSE_DAX_FMAP in perpetuity.

Setting CONFIG_FUSE_DAX=n does not mean userland "is broken", this would
only be the case if FUSE_DAX_FMAP was advertised but not actually
supported.

If DAX were removed from the kernel (unlikely, but stick with me) this
would be equivalent to permanently changing CONFIG_FUSE_DAX to always
off, and there would be no squabbles over whether that particular
change broke userland (there would be much strife over removing dax).

While not a deprecation method, this is what capability bits are
designed for. Same as cpuid capability bits - just because the bit is
there doesn't mean a processor is required to support it in perpetuity.

They're only required to support it if the bit is turned on.

---

I think the focus here needs to be on whether this interface ACTUALLY
needs to be more generic - and whether that is actually FEASIBLE.

It's not like this is a new problem - and there are real design reasons
why John chose this route.

The additional overhead is not trivial for FAMFS - FAMFS is not doing
i/o.  He already has data showing fuse caused a performance hit due to
overhead on open - his concern of overhead on fault being catastrophic
is grounded in data.

For others it's an age old problem of self-describing protocols (parsing
vs giant inflexible binary blobs, pick your poison).  It's extremely
unlikely we will find a one-size-fits-all solution that doesn't
eventually run right back into this same problem.

I worry that this discussion is going to turn towards implementing a
solution grounded in parsing arbitrary formats and how to store them,
and that is completely detached from why FAMFS went this route in the
first place.

I question whether the actual issue here lies in the interface APPEARING
more general purpose than it actually is - and therefore inviting
attempts to over-genericize it.

Is there a world here where this is solved by a name change and a
capability bit?  I think so.

~Gregory