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

[Edward Shishkin <edward.shishkin@gmail.com>]

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.

> 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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