Re: [PATCH RFC v2 19/27] mm: mprotect: Introduce PAGE_FAULT_ON_ACCESS for mprotect(PROT_MTE)

[David Hildenbrand]

On 29.11.23 12:55, Alexandru Elisei wrote:
> Hi,
>
> On Tue, Nov 28, 2023 at 06:55:18PM +0100, David Hildenbrand wrote:
> > On 19.11.23 17:57, Alexandru Elisei wrote:
> > > To enable tagging on a memory range, userspace can use mprotect() with the
> > > PROT_MTE access flag. Pages already mapped in the VMA don't have the
> > > associated tag storage block reserved, so mark the PTEs as
> > > PAGE_FAULT_ON_ACCESS to trigger a fault next time they are accessed, and
> > > reserve the tag storage on the fault path.
> >
> > That sounds alot like fake PROT_NONE. Would there be a way to unify hat
>
> Yes, arm64 basically defines PAGE_FAULT_ON_ACCESS as PAGE_NONE |
> PTE_TAG_STORAGE_NONE.
>
> > handling and simply reuse pte_protnone()? For example, could we special case
> > on VMA flags?
> >
> > Like, don't do NUMA hinting in these special VMAs. Then, have something
> > like:
> >
> > if (pte_protnone(vmf->orig_pte))
> > 	return handle_pte_protnone(vmf);
> >
> > In there, special case on the VMA flags.
>
> Your suggestion from the follow-up reply that an arch should know if it needs to
> do something was spot on, arm64 can use the software bit in the translation
> table entry for that.
>
> So what you are proposing is this:
>
> * Rename do_numa_page->handle_pte_protnone
> * At some point in the do_numa_page (now renamed to handle_pte_protnone) flow,
>    decide if pte_protnone() has been set for an arch specific reason or because
>    of automatic NUMA balancing.
> * if pte_protnone() has been set by an architecture, then let the architecture
>    handle the fault.
>
> If I understood you correctly, that's a good idea, and should be easy to
> implement.

yes! :)

--
Cheers,

David / dhildenb