Re: [LSF/MM/BPF TOPIC] Where is fuse going? API cleanup, restructuring and more

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Feb 06, 2026 at 02:48:43PM -0600, John Groves wrote:
> On 26/02/05 09:52PM, Darrick J. Wong wrote:
> > On Thu, Feb 05, 2026 at 10:27:52AM +0100, Amir Goldstein wrote:
> > > On Thu, Feb 5, 2026 at 4:33 AM John Groves <john@jagalactic.com> wrote:
> > > >
> > > > On 26/02/04 11:06AM, Darrick J. Wong wrote:
> > > >
> > > > [ ... ]
> > > >
> > > > > >  - famfs: export distributed memory
> > > > >
> > > > > This has been, uh, hanging out for an extraordinarily long time.
> > > >
> > > > Um, *yeah*. Although a significant part of that time was on me, because
> > > > getting it ported into fuse was kinda hard, my users and I are hoping we
> > > > can get this upstreamed fairly soon now. I'm hoping that after the 6.19
> > > > merge window dust settles we can negotiate any needed changes etc. and
> > > > shoot for the 7.0 merge window.
> > 
> > I think we've all missed getting merged for 7.0 since 6.19 will be
> > released in 3 days. :/
> > 
> > (Granted most of the maintainers I know are /much/ less conservative
> > than I was about the schedule)
> 
> Doh - right you are...
> 
> > 
> > > I think that the work on famfs is setting an example, and I very much
> > > hope it will be a good example, of how improving existing infrastructure
> > > (FUSE) is a better contribution than adding another fs to the pile.
> > 
> > Yeah.  Joanne and I spent a couple of days this week coprogramming a
> > prototype of a way for famfs to create BPF programs to handle
> > INTERLEAVED_EXTENT files.  We might be ready to show that off in a
> > couple of weeks, and that might be a way to clear up the
> > GET_FMAP/IOMAP_BEGIN logjam at last.
> 
> I'd love to learn more about this; happy to do a call if that's a
> good way to get me briefed.
> 
> I [generally but not specifically] understand how this could avoid
> GET_FMAP, but not GET_DAXDEV. 

fuse-iomap requires fuse servers to open block devices and register them
with the fuse_conn as a backing file.  The kernel returns a magic cookie
that can then be passed back to the kernel in iomap_begin.  This is
(AFAICT) similar to what fuse does w.r.t. passthrough files.

IIRC, GET_DAXDEV is an ondemand fuse request, which is quite different
from the fuse-iomap model where bdevs have to be registered before you
can use them.

> But I'm not sure it could (or should) avoid dax_iomap_rw() and
> dax_iomap_fault(). The thing is that those call my begin() function
> to resolve an offset in a file to an offset on a daxdev, and then
> dax completes the fault or memcpy. In that dance, famfs never knows
> the kernel address of the memory at all (also true of xfs in fs-dax
> mode, unless that's changed fairly recently). I think that's a pretty
> decent interface all in all.

Right.  dax_iomap_{rw,fault} call the ->iomap_begin they're given, which
can be fuse_iomap_begin, which will either (a) look in the iext cache,
(b) see if the fuse server supplied a bpf program, or (c) upcall the
fuse server.

I also took another look at my broken fuse-iomap-dax patch and realized
that in addition to corrupting data somewhere, there's also a gigantic
XXX around dax_writeback_mapping_range because it takes a bdev instead
of asking the filesystem for mappings, which means that it's broken for
any fsdax file who stores data on more than one device.

> Also: dunno whether y'all have looked at the dax patches in the famfs
> series, but the solution to working with Alistair's folio-ification 
> and cleanup of the dax layer (which set me back months) was to create 
> drivers/dax/fsdev.c, which, when bound to a daxdev in place of 
> drivers/dax/device.c, configures folios & pages compatibly with 
> fs-dax. So I kinda think I need the dax_iomap* interface.

Oh that's good news!

--D

> As usual, if I'm overlooking something let me know...
> 
> Regards,
> John
> 
>