Re: [BUG] fallocate behavior when crossing end-of-file

Chris Mason
2009-05-06T15:10:23Z

On Wed, 2009-05-06 at 16:59 +0200, Edward Shishkin wrote:
> Raskin Michael wrote:
> > Hello.
> >
>
> Hello.
>
> > I found the following bug in BtrFS:
> >
> > 1. Create and open an empty file
> > 2. fallocate (fd, 0, 1)
> >
> > Desired: something (probably, one block) is allocated/reserved for the
> > file. File length is set to 1 byte.
> >
>
> Where is it documented?
>
> IMHO we should only guarantee that writes
> to [0, 1] won't fail because of lack of disk space.
> Other things (including file size) are up to implementation.
>

This one looks like a pretty simple bug in the fallocate call. It is
rounding up the size to a block boundary.

-chris

> > Actual: A block is allocated. File length is set to 1 block (4096
> > bytes). The rest of the file is filled with zeros.
> >
> > last_byte = min(extent_map_end(em), alloc_end);
> > last_byte = (last_byte + mask) & ~mask;
> > if (em->block_start == EXTENT_MAP_HOLE) {
> > ret = prealloc_file_range(trans, inode, cur_offset,
> > last_byte, locked_end + 1,
> > alloc_hint, mode);
> >
> > That part seems strange to me. You make an effort for block size to
> > divide last_byte. But last_byte should be
> > max(i_size_read(inode), offset+len)
> > - without any rounding.
> >
> > Michael Raskin
> > --
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