mm: introduce mm_forbids_zeropage function

Add a new function stub to allow architectures to disable for
an mm_structthe backing of non-present, anonymous pages with
read-only empty zero pages.

Signed-off-by: Dominik Dingel <[e-mail]>
Reviewed-by: Paolo Bonzini <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

include/linux/mm.h

···
 #define __pa_symbol(x)  __pa(RELOC_HIDE((unsigned long)(x), 0))
 #endif
 
+/*
+ * To prevent common memory management code establishing
+ * a zero page mapping on a read fault.
+ * This macro should be defined within <asm/pgtable.h>.
+ * s390 does this to prevent multiplexing of hardware bits
+ * related to the physical page in case of virtualization.
+ */
+#ifndef mm_forbids_zeropage
+#define mm_forbids_zeropage(X)	(0)
+#endif
+
 extern unsigned long sysctl_user_reserve_kbytes;
 extern unsigned long sysctl_admin_reserve_kbytes;
 

mm/huge_memory.c

···
 		return VM_FAULT_OOM;
 	if (unlikely(khugepaged_enter(vma)))
 		return VM_FAULT_OOM;
-	if (!(flags & FAULT_FLAG_WRITE) &&
+	if (!(flags & FAULT_FLAG_WRITE) && !mm_forbids_zeropage(mm) &&
 			transparent_hugepage_use_zero_page()) {
 		spinlock_t *ptl;
 		pgtable_t pgtable;

mm/memory.c

···
 		return VM_FAULT_SIGBUS;
 
 	/* Use the zero-page for reads */
-	if (!(flags & FAULT_FLAG_WRITE)) {
+	if (!(flags & FAULT_FLAG_WRITE) && !mm_forbids_zeropage(mm)) {
 		entry = pte_mkspecial(pfn_pte(my_zero_pfn(address),
 						vma->vm_page_prot));
 		page_table = pte_offset_map_lock(mm, pmd, address, &ptl);