afs: Alter dirty range encoding in page->private

Currently, page->private on an afs page is used to store the range of
dirtied data within the page, where the range includes the lower bound, but
excludes the upper bound (e.g. 0-1 is a range covering a single byte).

This, however, requires a superfluous bit for the last-byte bound so that
on a 4KiB page, it can say 0-4096 to indicate the whole page, the idea
being that having both numbers the same would indicate an empty range.
This is unnecessary as the PG_private bit is clear if it's an empty range
(as is PG_dirty).

Alter the way the dirty range is encoded in page->private such that the
upper bound is reduced by 1 (e.g. 0-0 is then specified the same single
byte range mentioned above).

Applying this to both bounds frees up two bits, one of which can be used in
a future commit.

This allows the afs filesystem to be compiled on ppc32 with 64K pages;
without this, the following warnings are seen:

../fs/afs/internal.h: In function 'afs_page_dirty_to':
../fs/afs/internal.h:881:15: warning: right shift count >= width of type [-Wshift-count-overflow]
  881 |  return (priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK;
      |               ^~
../fs/afs/internal.h: In function 'afs_page_dirty':
../fs/afs/internal.h:886:28: warning: left shift count >= width of type [-Wshift-count-overflow]
  886 |  return ((unsigned long)to << __AFS_PAGE_PRIV_SHIFT) | from;
      |                            ^~

Fixes: 4343d00872 ("afs: Get rid of the afs_writeback record")
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/internal.h

@@ -863,7 +863,7 @@ struct afs_vnode_cache_aux {
  * splitting the field into two parts.  However, we need to represent a range
  * 0...PAGE_SIZE inclusive, so we can't support 64K pages on a 32-bit system.
  */
-#if PAGE_SIZE > 32768
+#ifdef CONFIG_64BIT
 #define __AFS_PAGE_PRIV_MASK	0xffffffffUL
 #define __AFS_PAGE_PRIV_SHIFT	32
 #else
@@ -878,12 +878,12 @@ static inline size_t afs_page_dirty_from(unsigned long priv)
 
 static inline size_t afs_page_dirty_to(unsigned long priv)
 {
-	return (priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK;
+	return ((priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK) + 1;
 }
 
 static inline unsigned long afs_page_dirty(size_t from, size_t to)
 {
-	return ((unsigned long)to << __AFS_PAGE_PRIV_SHIFT) | from;
+	return ((unsigned long)(to - 1) << __AFS_PAGE_PRIV_SHIFT) | from;
 }
 
 #include <trace/events/afs.h>

fs/afs/write.c

@@ -93,7 +93,7 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 	/* We want to store information about how much of a page is altered in
 	 * page->private.
 	 */
-	BUILD_BUG_ON(PAGE_SIZE > 32768 && sizeof(page->private) < 8);
+	BUILD_BUG_ON(PAGE_SIZE - 1 > __AFS_PAGE_PRIV_MASK && sizeof(page->private) < 8);
 
 	page = grab_cache_page_write_begin(mapping, index, flags);
 	if (!page)