Re: [PATCH v5 19/25] arm64/mm: Wire up PTE_CONT for user mappings

[David Hildenbrand]

On 12.02.24 21:38, Ryan Roberts wrote:
> [...]
>
> > > > > +static inline bool mm_is_user(struct mm_struct *mm)
> > > > > +{
> > > > > +	/*
> > > > > +	 * Don't attempt to apply the contig bit to kernel mappings, because
> > > > > +	 * dynamically adding/removing the contig bit can cause page faults.
> > > > > +	 * These racing faults are ok for user space, since they get serialized
> > > > > +	 * on the PTL. But kernel mappings can't tolerate faults.
> > > > > +	 */
> > > > > +	return mm != &init_mm;
> > > > > +}
> > > >
> > > > We also have the efi_mm as a non-user mm, though I don't think we manipulate
> > > > that while it is live, and I'm not sure if that needs any special handling.
> > >
> > > Well we never need this function in the hot (order-0 folio) path, so I think I
> > > could add a check for efi_mm here with performance implication. It's probably
> > > safest to explicitly exclude it? What do you think?
> >
> > Oops: This should have read "I think I could add a check for efi_mm here
> > *without* performance implication"
>
> It turns out that efi_mm is only defined when CONFIG_EFI is enabled. I can do this:
>
> return mm != &init_mm && (!IS_ENABLED(CONFIG_EFI) || mm != &efi_mm);

Please use all the lines you need ;)

if (IS_ENABLED(CONFIG_EFI) && unlikely(mm == &efi_mm))
	return false;
return mm != &init_mm;

> Is that acceptable? This is my preference, but nothing else outside of efi
> references this symbol currently.

We could also mark MMs in some way to be special.

return mm->is_user;

Then it's easy to extend.

--
Cheers,

David / dhildenb