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

On 15/06/25 1:02 pm, Mike Rapoport wrote:

On Fri, Jun 13, 2025 at 07:13:51PM +0530, Dev Jain wrote:

quoted

-/*
- * 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.

quoted

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