Reduce max bitmap size when bitmap is in a file.

To ensure kernel never needs to allocate more than 1 page contiguously.

ChangeLog

@@ -1,6 +1,9 @@
 Changes Prior to this release
     -   Don't #include linux/blkpg.h as that isn't safe.  Just
 	include the content literally.
+    -   Reduce maximum bitmap usage when working with bitmap files,
+	so that a only single-page allocations are made, even on
+	64bit hosts with 4K pages.
 
 Changes Prior to 2.5.4 release
     -   When creating devices in /dev/md/ create matching symlinks

bitmap.c

@@ -345,11 +345,12 @@ int CreateBitmap(char *filename, int force, char uuid[16],
 	if (chunksize == UnSet) {
 		/* We don't want more than 2^21 chunks, as 2^11 fill up one
 		 * 4K page (2 bytes per chunk), and 2^10 address of those
-		 * fill up a 4K indexing page.  2^20 might be safer...
+		 * fill up a 4K indexing page.  2^20 might be safer, especially
+		 * on 64bit hosts, so use that.
 		 */
 		chunksize = DEFAULT_BITMAP_CHUNK;
-		/* <<21 for 2^21 chunks, >>9 to convert bytes to sectors */
-		while (array_size > (chunksize << (21-9)))
+		/* <<20 for 2^20 chunks, >>9 to convert bytes to sectors */
+		while (array_size > (chunksize << (20-9)))
 			chunksize <<= 1;
 	}