Comment 7 for bug 474597

Sorry for the delay in attending to this patch. This is what I've
added to the ext4 patch queue.

      - Ted

commit 7f9aeb212e41ea20f61061d51ad11ee2449853f1
Author: Theodore Ts'o <email address hidden>
Date: Mon Feb 15 14:48:32 2010 -0500

    ext4: correctly calculate number of blocks for fiemap

    ext4_fiemap() rounds the length of the requested range down to
    blocksize, which is is not the true number of blocks that cover the
    requested region. This problem is especially impressive if the user
    requests only the first byte of a file: not a single extent will be
    reported.

    We fix this by calculating the last block of the region and then
    subtract to find the number of blocks in the extents.

    Signed-off-by: Leonard Michlmayr <email address hidden>
    Signed-off-by: "Theodore Ts'o" <email address hidden>

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index bd80891..bc9860f 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3768,7 +3768,6 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
   __u64 start, __u64 len)
 {
  ext4_lblk_t start_blk;
- ext4_lblk_t len_blks;
  int error = 0;

  /* fallback to generic here if not in extents fmt */
@@ -3782,8 +3781,11 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
  if (fieinfo->fi_flags & FIEMAP_FLAG_XATTR) {
   error = ext4_xattr_fiemap(inode, fieinfo);
  } else {
+ ext4_lblk_t last_blk, len_blks;
+
   start_blk = start >> inode->i_sb->s_blocksize_bits;
- len_blks = len >> inode->i_sb->s_blocksize_bits;
+ last_blk = (start + len - 1) >> inode->i_sb->s_blocksize_bits;
+ len_blks = last_blk - start_blk + 1;

   /*
    * Walk the extent tree gathering extent information.