Define functions for page cache handling

We use the macros PAGE_CACHE_SIZE PAGE_CACHE_SHIFT PAGE_CACHE_MASK
and PAGE_CACHE_ALIGN in various places in the kernel. Many times
common operations like calculating the offset or the index are coded
using shifts and adds. 

These are useful if one only want to support one page size in the page cache.

This patch provides a set of functions in order to provide the
ability to define new page size in the future.

All functions take an address_space pointer. Add a set of extended functions
that will be used to consolidate the hand crafted shifts and adds in use
right now.

New function			Related base page constant
---------------------------------------------------
page_cache_shift(a)		PAGE_CACHE_SHIFT
page_cache_size(a)		PAGE_CACHE_SIZE
page_cache_mask(a)		PAGE_CACHE_MASK
page_cache_index(a, pos)	Calculate page number from position
page_cache_next(addr, pos)	Page number of next page
page_cache_offset(a, pos)	Calculate offset into a page
page_cache_pos(a, index, offset)
				Form position based on page number
				and an offset.

The workings of these functions depend on CONFIG_LARGE_BLOCKSIZE. If set
then these sizes are dynamically calculated. Otherwise the functions will
provide constant results.


Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/pagemap.h |   54 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

Index: vps/include/linux/pagemap.h
===================================================================
--- vps.orig/include/linux/pagemap.h	2007-06-08 10:57:49.000000000 -0700
+++ vps/include/linux/pagemap.h	2007-06-08 11:01:37.000000000 -0700
@@ -52,12 +52,66 @@ static inline void mapping_set_gfp_mask(
  * space in smaller chunks for same flexibility).
  *
  * Or rather, it _will_ be done in larger chunks.
+ *
+ * The following constants can be used if a filesystem only supports a single
+ * page size.
  */
 #define PAGE_CACHE_SHIFT	PAGE_SHIFT
 #define PAGE_CACHE_SIZE		PAGE_SIZE
 #define PAGE_CACHE_MASK		PAGE_MASK
 #define PAGE_CACHE_ALIGN(addr)	(((addr)+PAGE_CACHE_SIZE-1)&PAGE_CACHE_MASK)
 
+/*
+ * Functions that are currently setup for a fixed PAGE_SIZEd. The use of
+ * these will allow a variable page size pagecache in the future.
+ */
+static inline int mapping_order(struct address_space *a)
+{
+	return 0;
+}
+
+static inline int page_cache_shift(struct address_space *a)
+{
+	return PAGE_SHIFT;
+}
+
+static inline unsigned int page_cache_size(struct address_space *a)
+{
+	return PAGE_SIZE;
+}
+
+static inline loff_t page_cache_mask(struct address_space *a)
+{
+	return (loff_t)PAGE_MASK;
+}
+
+static inline unsigned int page_cache_offset(struct address_space *a,
+		loff_t pos)
+{
+	return pos & ~PAGE_MASK;
+}
+
+static inline pgoff_t page_cache_index(struct address_space *a,
+		loff_t pos)
+{
+	return pos >> page_cache_shift(a);
+}
+
+/*
+ * Index of the page starting on or after the given position.
+ */
+static inline pgoff_t page_cache_next(struct address_space *a,
+		loff_t pos)
+{
+	return page_cache_index(a, pos + page_cache_size(a) - 1);
+}
+
+static inline loff_t page_cache_pos(struct address_space *a,
+		pgoff_t index, unsigned long offset)
+{
+	return ((loff_t)index << page_cache_shift(a)) + offset;
+}
+
 #define page_cache_get(page)		get_page(page)
 #define page_cache_release(page)	put_page(page)
 void release_pages(struct page **pages, int nr, int cold);