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    SubjectRe: ext4_fiemap gives 0 extents for files smaller than a block (patch included)
    On 2009-11-04, at 11:42, Leonard Michlmayr wrote:
    > Fiemap (ioctl) does not return any extents for small files on ext4.
    > (fm_start=0, fm_length=filesize)
    >
    > File affected: fs/ext4/extents.c
    >
    > I found the reason of the bug: wrong rounding. It will not only affect
    > small files, but any request that overlaps an extent boundary by less
    > that blocksize.

    >
    > @@ -3700,7 +3701,8 @@
    > start_blk = start >> inode->i_sb->s_blocksize_bits;
    > - len_blks = len >> inode->i_sb->s_blocksize_bits;
    > + end_blk = (start + len - 1) >> inode->i_sb->s_blocksize_bits;
    > + len_blks = end_blk - start_blk + 1;

    I don't think this is quite correct either. For example, if blocksize
    is 1024
    and start is 1023 (start_blk = 0) and len is 2 (end = 1024, end_blk =
    1) then
    len_blks = 2 which is too much.

    I think the right calculation here is:

    end_blk = (start + len + inode->i_sb->s_blocksize -
    1) >>
    inode->i_sb->s_blocksize_bits;
    len_blks = end_blk - start_blk;

    I'm also wondering (unrelated to this bug) why inode->i_sb-
    >s_blocksize_bits
    is used instead of inode->i_blkbits? That is probably worth a
    separate cleanup
    patch.

    Cheers, Andreas
    --
    Andreas Dilger
    Sr. Staff Engineer, Lustre Group
    Sun Microsystems of Canada, Inc.



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