mm: fix __page_to_pfn for a const struct page argument

This allows the cast in lowmem_page_address (introduced as a warning
fixup to 33dd4e0ec911 "mm: make some struct page's const") to be
removed.

Propagate const'ness to page_to_section() as well since it is required
by __page_to_pfn.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/memory_model.h

@@ -39,7 +39,7 @@
 })
 
 #define __page_to_pfn(pg)						\
-({	struct page *__pg = (pg);					\
+({	const struct page *__pg = (pg);					\
 	struct pglist_data *__pgdat = NODE_DATA(page_to_nid(__pg));	\
 	(unsigned long)(__pg - __pgdat->node_mem_map) +			\
 	 __pgdat->node_start_pfn;					\
@@ -57,7 +57,7 @@
  * section[i].section_mem_map == mem_map's address - start_pfn;
  */
 #define __page_to_pfn(pg)					\
-({	struct page *__pg = (pg);				\
+({	const struct page *__pg = (pg);				\
 	int __sec = page_to_section(__pg);			\
 	(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec)));	\
 })

include/linux/mm.h

@@ -685,7 +685,7 @@ static inline void set_page_section(struct page *page, unsigned long section)
 	page->flags |= (section & SECTIONS_MASK) << SECTIONS_PGSHIFT;
 }
 
-static inline unsigned long page_to_section(struct page *page)
+static inline unsigned long page_to_section(const struct page *page)
 {
 	return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
 }
@@ -720,7 +720,7 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
 
 static __always_inline void *lowmem_page_address(const struct page *page)
 {
-	return __va(PFN_PHYS(page_to_pfn((struct page *)page)));
+	return __va(PFN_PHYS(page_to_pfn(page)));
 }
 
 #if defined(CONFIG_HIGHMEM) && !defined(WANT_PAGE_VIRTUAL)