a question regarding Ext3 file truncate

a question regarding Ext3 file truncate

[Xin Zhao]

Hi,

Please forgive me if the question is dumb.

I am modifying ext3 to add some new features, but was confused by the
implementation of ext3_truncate().

In ext3_truncate():

we first use
	n = ext3_block_to_path(inode, last_block, offsets, NULL);
to get the path of the last block.

If the number of blocks is smaller than 12,  all blocks are then
direct blocks. We then need to clear them.

But the interesting thing happens:
	if (n == 1)									  /* direct blocks */
	{
		ext3_free_data(handle, inode, NULL, i_data+offsets[0],
			i_data + EXT3_NDIR_BLOCKS);
		goto do_indirects;
	}
This code seems to free data blocks right after the blocks used by the
file. I think it should be
ext3_free_data(handle, inode, NULL, i_data, i_data+offsets[0]);

Right? can some one explain this a little bit?

Thanks,
-x

Re: a question regarding Ext3 file truncate

[Mingming Cao]

On Wed, 2007-02-07 at 12:16 -0500, Xin Zhao wrote:
> Hi,
> 
> Please forgive me if the question is dumb.
> 
> I am modifying ext3 to add some new features, but was confused by the
> implementation of ext3_truncate().
> 
> In ext3_truncate():
> 
> we first use
> 	n = ext3_block_to_path(inode, last_block, offsets, NULL);
> to get the path of the last block.
> 
> If the number of blocks is smaller than 12,  all blocks are then
> direct blocks. We then need to clear them.
> 
> But the interesting thing happens:
> 	if (n == 1)									  /* direct blocks */
> 	{
> 		ext3_free_data(handle, inode, NULL, i_data+offsets[0],
> 			i_data + EXT3_NDIR_BLOCKS);
> 		goto do_indirects;
> 	}
> This code seems to free data blocks right after the blocks used by the
> file. I think it should be
> ext3_free_data(handle, inode, NULL, i_data, i_data+offsets[0]);

Last_block is the last logical block after the truncate, so
ext3_truncate() free data blocks after this point.

Mingming

Re: a question regarding Ext3 file truncate

[Xin Zhao]

Thanks a lot! I just found it.

If we need to truncate a file to 0 bytes, we have to change i_size to
0 first, then call ext3_truncate(). Then the code makes sense.

Xin

On 2/7/07, Mingming Cao <cmm@us.ibm.com> wrote:
> On Wed, 2007-02-07 at 12:16 -0500, Xin Zhao wrote:
> > Hi,
> >
> > Please forgive me if the question is dumb.
> >
> > I am modifying ext3 to add some new features, but was confused by the
> > implementation of ext3_truncate().
> >
> > In ext3_truncate():
> >
> > we first use
> >       n = ext3_block_to_path(inode, last_block, offsets, NULL);
> > to get the path of the last block.
> >
> > If the number of blocks is smaller than 12,  all blocks are then
> > direct blocks. We then need to clear them.
> >
> > But the interesting thing happens:
> >       if (n == 1)                                                                       /* direct blocks */
> >       {
> >               ext3_free_data(handle, inode, NULL, i_data+offsets[0],
> >                       i_data + EXT3_NDIR_BLOCKS);
> >               goto do_indirects;
> >       }
> > This code seems to free data blocks right after the blocks used by the
> > file. I think it should be
> > ext3_free_data(handle, inode, NULL, i_data, i_data+offsets[0]);
>
> Last_block is the last logical block after the truncate, so
> ext3_truncate() free data blocks after this point.
>
> Mingming
>
>