[XFS] Fixed a few bugs in xfs_buf_associate_memory()

- calculation of 'page_count' was incorrect as it did not
  consider the offset of 'mem' into the first page. The
  logic to bump 'page_count' didn't work if 'len' was <=
  PAGE_CACHE_SIZE (ie offset = 3k, len = 2k).
- setting b_buffer_length to 'len' is incorrect if 'offset'
  is > 0. Set it to the total length of the buffer.
- I suspect that passing a non-aligned address into
  mem_to_page() for the first page may have been causing
  issues - don't know but just tidy up that code anyway.

SGI-PV: 971596
SGI-Modid: xfs-linux-melb:xfs-kern:30143a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>

fs/xfs/linux-2.6/xfs_buf.c

@@ -725,15 +725,15 @@ xfs_buf_associate_memory(
 {
 	int			rval;
 	int			i = 0;
-	size_t			ptr;
-	size_t			end, end_cur;
-	off_t			offset;
+	unsigned long		pageaddr;
+	unsigned long		offset;
+	size_t			buflen;
 	int			page_count;
 
-	page_count = PAGE_CACHE_ALIGN(len) >> PAGE_CACHE_SHIFT;
-	offset = (off_t) mem - ((off_t)mem & PAGE_CACHE_MASK);
-	if (offset && (len > PAGE_CACHE_SIZE))
-		page_count++;
+	pageaddr = (unsigned long)mem & PAGE_CACHE_MASK;
+	offset = (unsigned long)mem - pageaddr;
+	buflen = PAGE_CACHE_ALIGN(len + offset);
+	page_count = buflen >> PAGE_CACHE_SHIFT;
 
 	/* Free any previous set of page pointers */
 	if (bp->b_pages)
@@ -747,22 +747,15 @@ xfs_buf_associate_memory(
 		return rval;
 
 	bp->b_offset = offset;
-	ptr = (size_t) mem & PAGE_CACHE_MASK;
-	end = PAGE_CACHE_ALIGN((size_t) mem + len);
-	end_cur = end;
-	/* set up first page */
-	bp->b_pages[0] = mem_to_page(mem);
-
-	ptr += PAGE_CACHE_SIZE;
-	bp->b_page_count = ++i;
-	while (ptr < end) {
-		bp->b_pages[i] = mem_to_page((void *)ptr);
-		bp->b_page_count = ++i;
-		ptr += PAGE_CACHE_SIZE;
+
+	for (i = 0; i < bp->b_page_count; i++) {
+		bp->b_pages[i] = mem_to_page((void *)pageaddr);
+		pageaddr += PAGE_CACHE_SIZE;
 	}
 	bp->b_locked = 0;
 
-	bp->b_count_desired = bp->b_buffer_length = len;
+	bp->b_count_desired = len;
+	bp->b_buffer_length = buflen;
 	bp->b_flags |= XBF_MAPPED;
 
 	return 0;