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

[Raskin Michael <raskin@mccme.ru>]

		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.

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