[PATCH] remove page->virtual

The patch removes page->virtual for all architectures which do not
define WANT_PAGE_VIRTUAL.  Hash for it instead.

Possibly we could define WANT_PAGE_VIRTUAL for CONFIG_HIGHMEM4G, but it
seems unlikely.

A lot of the pressure went off kmap() and page_address() as a result of
the move to kmap_atomic().  That should be the preferred way to address
CPU load in the set_page_address() and page_address() hashing and
locking.

If kmap_atomic is not usable then the next best approach is for users
to cache the result of kmap() in a local rather than calling
page_address() repeatedly.

One heavy user of kmap() and page_address() is the ext2 directory code.

On a 7G Quad PIII, running four concurrent instances of

	while true
	do
		find /usr/src/linux > /dev/null
	done

on ext2 with everything cached, profiling shows that the new hashed
set_page_address() and page_address() implementations consume 0.4% and
1.3% of CPU time respectively.   I think that's OK.

1 files changed, 22 insertions, 26 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4ae8eb10dcb2..a5107b5043f7 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -176,7 +176,7 @@ struct page {
 	 * Architectures with slow multiplication can define
 	 * WANT_PAGE_VIRTUAL in asm/page.h
 	 */
-#if defined(CONFIG_HIGHMEM) || defined(WANT_PAGE_VIRTUAL)
+#if defined(WANT_PAGE_VIRTUAL)
 	void *virtual;			/* Kernel virtual address (NULL if
 					   not kmapped, ie. highmem) */
 #endif /* CONFIG_HIGMEM || WANT_PAGE_VIRTUAL */
@@ -289,38 +289,34 @@ static inline void set_page_zone(struct page *page, unsigned long zone_num)
 	page->flags |= zone_num << ZONE_SHIFT;
 }
 
-/*
- * In order to avoid #ifdefs within C code itself, we define
- * set_page_address to a noop for non-highmem machines, where
- * the field isn't useful.
- * The same is true for page_address() in arch-dependent code.
- */
-#if defined(CONFIG_HIGHMEM) || defined(WANT_PAGE_VIRTUAL)
+#define lowmem_page_address(page)					\
+	__va( ( ((page) - page_zone(page)->zone_mem_map)		\
+			+ page_zone(page)->zone_start_pfn) << PAGE_SHIFT)
+
+#if defined(CONFIG_HIGHMEM) && !defined(WANT_PAGE_VIRTUAL)
+#define HASHED_PAGE_VIRTUAL
+#endif
 
+#if defined(WANT_PAGE_VIRTUAL)
+#define page_address(page) ((page)->virtual)
 #define set_page_address(page, address)			\
 	do {						\
 		(page)->virtual = (address);		\
 	} while(0)
+#define page_address_init()  do { } while(0)
+#endif
 
-#else /* CONFIG_HIGHMEM || WANT_PAGE_VIRTUAL */
-#define set_page_address(page, address)  do { } while(0)
-#endif /* CONFIG_HIGHMEM || WANT_PAGE_VIRTUAL */
-
-/*
- * Permanent address of a page. Obviously must never be
- * called on a highmem page.
- */
-#if defined(CONFIG_HIGHMEM) || defined(WANT_PAGE_VIRTUAL)
-
-#define page_address(page) ((page)->virtual)
-
-#else /* CONFIG_HIGHMEM || WANT_PAGE_VIRTUAL */
-
-#define page_address(page)						\
-	__va( ( ((page) - page_zone(page)->zone_mem_map)		\
-			+ page_zone(page)->zone_start_pfn) << PAGE_SHIFT)
+#if defined(HASHED_PAGE_VIRTUAL)
+void *page_address(struct page *page);
+void set_page_address(struct page *page, void *virtual);
+void page_address_init(void);
+#endif
 
-#endif /* CONFIG_HIGHMEM || WANT_PAGE_VIRTUAL */
+#if !defined(HASHED_PAGE_VIRTUAL) && !defined(WANT_PAGE_VIRTUAL)
+#define page_address(page) lowmem_page_address(page)
+#define set_page_address(page, address)  do { } while(0)
+#define page_address_init()  do { } while(0)
+#endif
 
 /*
  * Return true if this page is mapped into pagetables.  Subtle: test pte.direct