Comment 5 for bug 474597

Problem:
fs/ext4/extents.c:ext4_fiemap rounds the length of the requested range
down to blocksize. This 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.

Solution:
Calculate the last byte of the region and round to blocksize. Then get
the number of blocks by subtracting last_blk - start_blk and adding 1
for the first block. (The variable last_blk is introduced just for
easier reading.) This patch will fix this.

I already suggested this patch some time ago, this is the same patch for
a more recent kernel version.

Signed-off-by: Leonard Michlmayr <email address hidden>

diff -rup linux-2.6.32.7/fs/ext4/extents.c linux-2.6.32.7-lm/fs/ext4/extents.c
--- linux-2.6.32.7/fs/ext4/extents.c 2010-01-29 00:06:20.000000000 +0100
+++ linux-2.6.32.7-lm/fs/ext4/extents.c 2010-02-11 21:26:40.000000000 +0100
@@ -3711,6 +3711,7 @@ int ext4_fiemap(struct inode *inode, str
   __u64 start, __u64 len)
 {
  ext4_lblk_t start_blk;
+ ext4_lblk_t last_blk;
  ext4_lblk_t len_blks;
  int error = 0;

@@ -3726,7 +3727,9 @@ int ext4_fiemap(struct inode *inode, str
   error = ext4_xattr_fiemap(inode, fieinfo);
  } else {
   start_blk = start >> inode->i_sb->s_blocksize_bits;
- len_blks = len >> inode->i_sb->s_blocksize_bits;
+ /* the last byte in the range is (start + len - 1) */
+ 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.