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

Gregory Price
2026-04-14T22:20:42Z

On Tue, Apr 14, 2026 at 11:57:40AM -0700, Darrick J. Wong wrote:
> >
> > I very strongly object to making this a prerequisite to merging. This
> > is an untested idea that will certainly delay us by at least a couple
> > of merge windows when products are shipping now, and the existing approach
> > has been in circulation for a long time. It is TOO LATE!!!!!!
>
...
>
> That said, you're clearly pissed at the goalposts changing yet again,
> and that's really not fair that we collectively keep moving them.
>

This seems a bit more than moving a goalpost.

We're now gating working software, for real working hardware, on a novel,
unproven BPF ops structure that controls page table mappings on page table
faults which would be used by exactly 1 user : FAMFS.

And that singular user is harmed because it turns an O(1) offset
calculation into a pointer chase - on the hottest path (every fault).

John is right to push back here.

---

That said - I'm looking at fs/fuse/famfs.c and I'm asking myself what in
here is actually famfs-specific. If you just s/FAMFS/DAX/g - the file
just reads like a simple DAX-iomap backend with optional striping.

Would it be reasonable to refactor the dax layer (and users) to
create an ops structure that becomes the basis for the BPF solution?

We don't even know what the whole BPF scope is, and it seems wholly
unfair to John's and his users to make that solely their problem (for
negative value!).

~Gregory