Re: [RFC PATCH 3/6] mm: nommu: fix an issue on map request to /dev/zero

On Thu, Aug 13, 2026 at 02:22:03PM +0100, Matthew Wilcox wrote:
> On Thu, Aug 13, 2026 at 09:19:47PM +0900, Greg Kroah-Hartman wrote:
> > On Thu, Aug 13, 2026 at 03:33:58PM +0900, Hajime Tazaki wrote:
> > > Upon a private file mapping request to /dev/zero, it calls
> > > kernel_read() in do_mmap_private(), getting a failure with the message
> > > like: "kernel reads not supported for file /dev/zero", which is because
> > > zero_fops defined in drivers/char/mem.c has both .read and .read_iter
> > > definitions.
> >
> > Do you actually use a no-mmu system?
>
> If you look at Hajime's contributions, you'll see they're far from an AI
> slopper.
>
> https://lore.kernel.org/linux-mm/?q=hajime+tazaki
>
> > > @@ -869,7 +886,11 @@ static vm_flags_t determine_vm_flags(struct file *file,
> > >
> > >  	vm_flags = calc_vm_prot_bits(prot, 0) | calc_vm_flag_bits(file, flags);
> > >
> > > -	if (!file) {
> > > +	/* private and file mapping will be marked anonymous later (do_mmap_private()).
> > > +	 * and /dev/zero is marked by them at .mmap_prepare,
> > > +	 * which should be _before_ this point.
> > > +	 */
> >
> > Wrong coding style for the comment, which is very typical of
> > LLM-generated stuff.  Always rewrite the output of these tools to
> > actually be sane.
>
> A lot of humans write comments like this too.  Indeed, it used to be
> the preferred style for net/

Yeah Hajime is one of the good guys and I don't think he's a schlopper.

(I do want to review his series, as usual time is against me :)

I do agree with Greg however that nommu is something we should get rid of. See
my other reply here.

Am frankly fed up with people telling us that nommu really matters and giving
talks at conferences but contributing nothing (apart from Hajime, who deserves
respect for walking the walk on that!)

If nommu can be broken for a year with nobody noticing, then maybe it's time to
see if anybody notices us removing it too.

Sadly I think Linus won't share this opinion but as co-maintainer of mm/nommu.c
this is my perspective ;)

--
Cheers, Lorenzo