mm: add kmap_to_page()

This is extracted from Mel Gorman's commit 5a178119b0 ('mm: add
support for direct_IO to highmem pages') upstream.

Required to backport commit b9cdc88df8 ('virtio: 9p: correctly pass
physical address to userspace for high pages').

Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

include/linux/highmem.h

@@ -38,10 +38,17 @@ extern unsigned long totalhigh_pages;
 
 void kmap_flush_unused(void);
 
+struct page *kmap_to_page(void *addr);
+
 #else /* CONFIG_HIGHMEM */
 
 static inline unsigned int nr_free_highpages(void) { return 0; }
 
+static inline struct page *kmap_to_page(void *addr)
+{
+	return virt_to_page(addr);
+}
+
 #define totalhigh_pages 0UL
 
 #ifndef ARCH_HAS_KMAP

mm/highmem.c

@@ -94,6 +94,18 @@ static DECLARE_WAIT_QUEUE_HEAD(pkmap_map_wait);
 		do { spin_unlock(&kmap_lock); (void)(flags); } while (0)
 #endif
 
+struct page *kmap_to_page(void *vaddr)
+{
+	unsigned long addr = (unsigned long)vaddr;
+
+	if (addr >= PKMAP_ADDR(0) && addr <= PKMAP_ADDR(LAST_PKMAP)) {
+		int i = (addr - PKMAP_ADDR(0)) >> PAGE_SHIFT;
+		return pte_page(pkmap_page_table[i]);
+	}
+
+	return virt_to_page(addr);
+}
+
 static void flush_all_zero_pkmaps(void)
 {
 	int i;