mm: arm64: document why pte is not advanced in contpte_ptep_set_access_flags()

According to David and Ryan, there isn't a bug here, even though we
don't advance the PTE entry, because __ptep_set_access_flags() only
uses the access flags from the entry.

However, we always check pte_same(pte, entry) using the first entry
in __ptep_set_access_flags(). This means that the checks from 1 to
nr - 1 are not comparing the same PTE indexes (thus, they always
return false), which can be a bit confusing. To clarify the code, let's
add some comments.

Reviewed-by: Ryan Roberts <[email protected]>
Signed-off-by: Barry Song <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Will Deacon <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: Barry Song

arch/arm64/mm/contpte.c

@@ -421,6 +421,12 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
 		ptep = contpte_align_down(ptep);
 		start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE);
 
+		/*
+		 * We are not advancing entry because __ptep_set_access_flags()
+		 * only consumes access flags from entry. And since we have checked
+		 * for the whole contpte block and returned early, pte_same()
+		 * within __ptep_set_access_flags() is likely false.
+		 */
 		for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE)
 			__ptep_set_access_flags(vma, addr, ptep, entry, 0);