Comment 10 for bug 474597

Ted, thanks for the notification.
The patch posted above is different than the one in the linux kernel.

commit aca92ff6f57c000d1b4523e383c8bd6b8269b8b1
Author: Leonard Michlmayr <email address hidden>
Date: Thu Mar 4 17:07:28 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..7d54850 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 +3781,14 @@ 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 len_blks;
+ __u64 last_blk;
+
                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;
+ if (last_blk >= EXT_MAX_BLOCK)
+ last_blk = EXT_MAX_BLOCK-1;
+ len_blks = ((ext4_lblk_t) last_blk) - start_blk + 1;

                /*
                 * Walk the extent tree gathering extent information.

The difference being the following two lines are added:
+ if (last_blk >= EXT_MAX_BLOCK)
+ last_blk = EXT_MAX_BLOCK-1;

* This is just a note to point the difference.