linux-kernel - Re: ext4_fiemap gives 0 extents for files smaller than a block (patch included)

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Message-ID: <1264446264.27189.3.camel@michlmayr>
Date:	Mon, 25 Jan 2010 20:04:24 +0100
From:	Leonard Michlmayr <leonard.michlmayr@...il.com>
To:	Andreas Dilger <adilger@....com>
Cc:	474597@...s.launchpad.net, Theodore Ts'o <tytso@....edu>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: ext4_fiemap gives 0 extents for files smaller than a block
 (patch included)

Do you have any news on this bug?

Will you apply the patch? Do you not plan to apply the patch?
Do you need an update of the patch?

Regards
Leonard

Am Mittwoch, den 04.11.2009, 12:44 -0700 schrieb Andreas Dilger:
> 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.
...

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