powerpc: Fix usage of _PAGE_RO in hugepage

On some CPUs like the 8xx, _PAGE_RW hence _PAGE_WRITE is defined
as 0 and _PAGE_RO has to be set when a page is not writable

_PAGE_RO is defined by default in pte-common.h, however BOOK3S/64
doesn't include that file so _PAGE_RO has to be defined explicitly
in book3s/64/pgtable.h

Fixes: a7b9f671f2d14 ("powerpc32: adds handling of _PAGE_RO")
Signed-off-by: Christophe Leroy <[e-mail]>
Reviewed-by: Aneesh Kumar K.V <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/include/asm/book3s/64/pgtable.h

···
  */
 #define _PAGE_BIT_SWAP_TYPE	0
 
+#define _PAGE_RO		0
+
 #define _PAGE_EXEC		0x00001 /* execute permission */
 #define _PAGE_WRITE		0x00002 /* write access allowed */
 #define _PAGE_READ		0x00004	/* read access allowed */

arch/powerpc/mm/hugetlbpage.c

···
 
 	pte = READ_ONCE(*ptep);
 	mask = _PAGE_PRESENT | _PAGE_READ;
+
+	/*
+	 * On some CPUs like the 8xx, _PAGE_RW hence _PAGE_WRITE is defined
+	 * as 0 and _PAGE_RO has to be set when a page is not writable
+	 */
 	if (write)
 		mask |= _PAGE_WRITE;
+	else
+		mask |= _PAGE_RO;
 
 	if ((pte_val(pte) & mask) != mask)
 		return 0;