Re: [Gnu-arch-users] Re: give us a hand with arch

[Florian Weimer]

On Mon, Sep 29, 2003 at 10:40:49PM +0200, Robin Farine wrote:

> >>>>> "Florian" == Florian Weimer <address@hidden> writes:
> 
>     Florian> Hans Reiser once told me that they won't overwrite unused
>     Florian> disk space with zeros, even when it might become visible
>     Florian> using mmap() -- for performance reasons.
> 
> I don't follow you. What do mean by "unused disk space becoming
> visible using mmap()"? mmap() applied to a regular file can only
> access the area corresponding to the contents of the file, and thus it
> cannot access unused disk space, no?

The last block of a file contains data after the actual end of the file
(back then, ReiserFS didn't optimize these cases).  This data is also
present in the page cache.  You won't see it normally, because you
cannot read() beyond the end of the file, but mmap() can only map
complete pages, so you suddenly see the data (and it was random data
from kernel or user space, typically not something which ever was on
disk).