mm/mmap: clarify protection_map[] indices

protection_map[] maps vm_flags access combinations into page protection
value as defined by the platform via __PXXX and __SXXX macros.  The array
indices in protection_map[], represents vm_flags access combinations but
it's not very intuitive to derive.  This makes it clear and explicit.

Link: https://lkml.kernel.org/r/20220404031840.588321-3-anshuman.khandual@arm.comSigned-off-by: NAnshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -102,8 +102,22 @@ static void unmap_region(struct mm_struct *mm,
  *								x: (yes) yes
  */
 pgprot_t protection_map[16] __ro_after_init = {
-	__P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111,
-	__S000, __S001, __S010, __S011, __S100, __S101, __S110, __S111
+	[VM_NONE]					= __P000,
+	[VM_READ]					= __P001,
+	[VM_WRITE]					= __P010,
+	[VM_WRITE | VM_READ]				= __P011,
+	[VM_EXEC]					= __P100,
+	[VM_EXEC | VM_READ]				= __P101,
+	[VM_EXEC | VM_WRITE]				= __P110,
+	[VM_EXEC | VM_WRITE | VM_READ]			= __P111,
+	[VM_SHARED]					= __S000,
+	[VM_SHARED | VM_READ]				= __S001,
+	[VM_SHARED | VM_WRITE]				= __S010,
+	[VM_SHARED | VM_WRITE | VM_READ]		= __S011,
+	[VM_SHARED | VM_EXEC]				= __S100,
+	[VM_SHARED | VM_EXEC | VM_READ]			= __S101,
+	[VM_SHARED | VM_EXEC | VM_WRITE]		= __S110,
+	[VM_SHARED | VM_EXEC | VM_WRITE | VM_READ]	= __S111
 };
 
 #ifndef CONFIG_ARCH_HAS_FILTER_PGPROT