make_page_readonly() must flush unused kmaps when handling

a high-memory page. Otherwise stale writable mappings may 
persist. This fixes a corner case for alloc_ldt (which can
vmalloc an LDT and hence use highmem pages).

Signed-off-by: Keir Fraser <keir@xensource.com>

linux-2.6-xen-sparse/arch/xen/i386/mm/pgtable.c

@@ -409,7 +409,9 @@ void make_page_readonly(void *va)
 	if ((unsigned long)va >= (unsigned long)high_memory) {
 		unsigned long pfn = pte_pfn(*pte);
 #ifdef CONFIG_HIGHMEM
-		if (pfn < highstart_pfn)
+		if (pfn >= highstart_pfn)
+			kmap_flush_unused(); /* flush stale writable kmaps */
+		else
 #endif
 			make_lowmem_page_readonly(
 				phys_to_virt(pfn << PAGE_SHIFT));