[PATCH] Fix up xfs_buf_associate_memory()

[Lachlan McIlroy <lachlan@xxxxxxx>]

Fixed a few bugs in xfs_buf_associate_memory() including:

- 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.

These fixes prevent an data corruption issue that can
occur during log replay.

Lachlan
--- fs/xfs/linux-2.6/xfs_buf.c_1.247    2007-11-23 12:03:16.000000000 +1100
+++ fs/xfs/linux-2.6/xfs_buf.c  2007-11-23 12:02:32.000000000 +1100
@@ -726,14 +726,14 @@ xfs_buf_associate_memory(
        int                     rval;
        int                     i = 0;
        size_t                  ptr;
-       size_t                  end, end_cur;
+       size_t                  buflen;
        off_t                   offset;
        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++;
+       ptr = (size_t) mem & PAGE_CACHE_MASK;
+       offset = (off_t) mem - (off_t) ptr;
+       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;
+
+       while (i < bp->b_page_count) {
+               bp->b_pages[i++] = mem_to_page((void *)ptr);
                ptr += 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;