Re: [PATCH v3 1/2] arm64: pageattr: Use pagewalk API to change memory permissions

[Dev Jain]

On 15/06/25 1:02 pm, Mike Rapoport wrote:
> On Fri, Jun 13, 2025 at 07:13:51PM +0530, Dev Jain wrote:
> > -/*
> > - * This function assumes that the range is mapped with PAGE_SIZE pages.
> > - */
> > -static int __change_memory_common(unsigned long start, unsigned long size,
> > +static int ___change_memory_common(unsigned long start, unsigned long size,
> >   				pgprot_t set_mask, pgprot_t clear_mask)
> >   {
> >   	struct page_change_data data;
> > @@ -61,9 +140,28 @@ static int __change_memory_common(unsigned long start, unsigned long size,
> >   	data.set_mask = set_mask;
> >   	data.clear_mask = clear_mask;
> >   
> > -	ret = apply_to_page_range(&init_mm, start, size, change_page_range,
> > -					&data);
> > +	arch_enter_lazy_mmu_mode();
> > +
> > +	/*
> > +	 * The caller must ensure that the range we are operating on does not
> > +	 * partially overlap a block mapping. Any such case should either not
> > +	 * exist, or must be eliminated by splitting the mapping - which for
> > +	 * kernel mappings can be done only on BBML2 systems.
> > +	 *
> > +	 */
> > +	ret = walk_kernel_page_table_range_lockless(start, start + size,
> > +						    &pageattr_ops, NULL, &data);
> x86 has a cpa_lock for set_memory/set_direct_map to ensure that there's on
> concurrency in kernel page table updates. I think arm64 has to have such
> lock as well.

My understanding is that it is guaranteed that the set_memory_* caller has
exclusive access to the range it is changing permissions for.
The x86 comment is

Serialize cpa() (for !DEBUG_PAGEALLOC which uses large identity mappings) using cpa_lock.
So that we don't allow any other cpu, with stale large tlb entries change the page attribute in
parallel to some other cpu splitting a large page entry along with changing the attribute.

On arm64 we are doing flush_tlb_kernel_range in __change_memory_common; and also, the caller
of __change_memory_common is required to first split the start and end before changing permissions,
so the splitting and permission change won't happen in parallel as described by the comment.

> > +	arch_leave_lazy_mmu_mode();
> > +
> > +	return ret;
> > +}