Re: [PATCH] mm: nommu: point to the write iterator upon split_vma

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, Jul 06, 2026 at 10:27:43AM +0200, Geert Uytterhoeven wrote:
> Hi Andrew,
>
> On Mon, 6 Jul 2026 at 07:23, Andrew Morton <akpm@linux-foundation.org> wrote:
> > On Mon, 6 Jul 2026 04:58:28 +0100 Matthew Wilcox <willy@infradead.org> wrote:
> > > On Sun, Jul 05, 2026 at 03:27:08PM -0700, Andrew Morton wrote:
> > > > Thanks.  Unfortunately we aren't very diligent about the nommu code
> > > > (are we?).   Perhaps appropriately - clearly this code doesn't get used a
> > > > lot.
> > >
> > > Should we delete support for NOMMU?  As you say, it doesn't get much
> > > testing, or presumably usage.  I have this quote from #m68k a couple of
> > > years ago...
> >
> > Greg is busily developing kernel code.
> >
> > > : I looked at nommu very briefly when I had my 68000 board on the bench
> > > : but didn’t get anywhere. Also didn’t try particularly hard. I think
> > > : you are better off with fuzix or OS9 if you want something unixy.
> > >
> > > (i suspect Zephyr would also be good, but it doesn't support m68k, just
> > > arc, arm, arm64, mips, openrisc, renesas rx, riscv, sparc, x86 and xtensa)
> >
> > Greg, how mush use is NOMMU Linux seeing nowadays?
>
> Nommu (SH-compatible J-Core ASIC) is used in actual products, running
> modern kernels.  The rationale is low interrupt latency without using
> a much more costly app CPU core + RT CPU core split.
>
> Please watch the recording of the "32bit and-or noMMU Linux BoF"[1]
> at LPC2025 for more info.

This crops up now and again and people point to uses, but fails to address the
real issue here.

Nobody (aside from Hajime - thank you!) who uses nommu maintains or contributes
to it, or _even tests_ it as far as I can tell.

We broken nommu for a year I think it was? And there wasn't one report.

The last time I raised this, I was admonished and told there are critical arm32
nommu devices that absolutely _must_ have the latest kernel and this is
perfectly working code.

And riscv also introduced (and I'm so very surprised Linus allowed it) a brand
new nommu architecture (!!)

And yet...

If this is really so important, can those who care perhaps help out a bit? At
least with testing?

I'm a bit fed up of it really.

As far as hobbyist retro stuff and upstream goes, I am sympathetic, I came from
being a hobbyist (though not retro), and would be one again if I couldn't do it
as a job.

But retro people do NOT need the latest kernel. As far as I'm concerned, museum
piece support should be ripped out, and those who care run downstream kernels.

As for the 'critical products' that must use nommu - PLEASE start
contributing. Please start testing. Anything.

Again thank you Hajime for doing the one thing that really counts here -
contributing to the nommu code.

Anyway, as to what Matthew said - 100% I am for us ripping it out.

But it's not up to me, and unfortunately I think we'll get a clammer of voices
saying how important it is, and silence again when it comes to do any of the
work.

>
> [1] https://lpc.events/event/19/contributions/2097/
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds

Thanks, Lorenzo