Re: Is _PAGE_PROTNONE set only for user mappings?

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, May 11, 2022 at 02:20:45PM +0900, Hyeonggon Yoo wrote:
> > pgprot_t vm_get_page_prot(unsigned long vm_flags)
> > {
> >        pgprot_t ret = __pgprot(pgprot_val(protection_map[vm_flags &
> >                                (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]) |
> >                        pgprot_val(arch_vm_get_page_prot(vm_flags)));
> >
> >        return arch_filter_pgprot(ret);
> > }
> > EXPORT_SYMBOL(vm_get_page_prot);
> 
> I guess it's only set for processes' VMA if no caller is abusing
> vm_get_page_prot() for kernel mappings.
> 
> But yeah, just quick guessing does not make us convinced.
> Let's Cc people working on mm.
> 
> If kernel never uses _PAGE_PROTNONE for kernel mappings, it's just okay
> not to clear _PAGE_GLOBAL at first in __change_page_attr() if it's not user address,
> because no user will confuse _PAGE_GLOBAL as _PAGE_PROTNONE if it's kernel
> address. right?
> 

I'm not aware of a case where _PAGE_BIT_PROTNONE is used for a kernel
address expecting PROT_NONE semantics instead of the global bit. NUMA
Balancing is not going to accidentally treat a kernel address as if it's
a NUMA hinting fault. By the time a fault is determining if a PTE access
is a numa hinting fault or accesssing a PROT_NONE region, it has been
established that it is a userspace address backed by a valid VMA.

-- 
Mel Gorman
SUSE Labs