Hi Lorenzo,
On Fri, 2 Oct 2026 at 10:10, Lorenzo Stoakes (ARM) <[email protected]> wrote:
> On Fri, Oct 02, 2026 at 08:08:44AM +0200, David Hildenbrand (Arm) wrote:
> > On 10/2/26 04:38, Hajime Tazaki wrote:
> > > On Thu, 01 Oct 2026 19:01:45 +0900,
> > > Johannes Berg wrote:
> > >>>
> > >>> I thought maybe uname could help but our LLM overlords tell me no.
> > >>
> > >> Hajime originally wanted uname to indicate it, but that caused
> > >> regressions elsewhere, so we dropped that commit. I guess we could put
> > >> _something_ back?
> > >
> > > systemd uses uname information and our (old) patch changing the format
> > > triggered the regression. thus, we dropped it.
> > >
> > >>> It pointed at something though.
> > >>>
> > >>> In fs/proc/meminfo.c:
> > >>>
> > >>> #ifndef CONFIG_MMU
> > >>> show_val_kb(m, "MmapCopy: ",
> > >>> (unsigned long)atomic_long_read(&mmap_pages_allocated));
> > >>> #endif
> > >>>
> > >>> So MmapCopy in /proc/meminfo tells us definitively, weirdly enough.
> > >>
> > >> Hyrum's law and all that, but I guess it's not highly likely to (want
> > >> to) change :)
> > >
> > > a runtime detection is nice.
> > >
> > > scanning /proc/meminfo might be useful. although it requires to mount
> > > procfs (we can actually skip procfs mount as it is optional) so, this
> > > is not 100% portable.
> > >
> > > checking ENOSYS like syscall(SYS_mprotect) might be useful but I
> > > wasn't sure if this is always the case of NOMMU or not.
> > >
> >
> > I assume there are plenty of other mechanisms. Like testing if fork isn't
> > allowed.
>
> Yup that's an obvious candidate!
>
> You actually need to check for both -EINVAL and -ENOSYS since, of course, we
> are
> inconsistent with this.
>
> Some nommu arches specify __ARCH_WANT_SYS_FORK unconditionally for both
> mmu/nommu variants meaning you get -EINVAL in that case:
>
> https://elixir.bootlin.com/linux/v7.2.8/source/kernel/fork.c#L2830
>
> Otherwise you get -ENOSYS because it's not defined.
>
> But fork() -> -EINVAL or -ENOSYS = nommu, then just have the child quit and
> etc. should do it.
fork() is not strictly an MMU-feature (you can implement fork on nommu
if all memory accesses are done relative to a base address register),
while I think mprotect() is an MMU-feature. Unless it can also be used
to change the protection using the one or two BARs most MCU haves?
Anything else that is a real MMU-feature? Non-identity mmap()?
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
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
