Messages in this thread | | | Date | Wed, 04 Nov 2009 12:44:11 -0700 | From | Andreas Dilger <> | Subject | Re: ext4_fiemap gives 0 extents for files smaller than a block (patch included) |
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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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