On Mon, Jul 30, 2007 at 11:14:33AM +0200, Joerg Sonnenberger wrote:
> On Tue, Jul 24, 2007 at 05:15:31PM -0500, Eric Haszlakiewicz wrote:
> >  if you map a 10 byte file (for example), the mmap man page says that
> > the mapped region may be extended up to the page size.
> >   I suppose there's a bit of a performance hit with zeroing out the page
> > every time, but it seems like a necessary thing to do.  A couple other
> > OSes I've tried it on do so.
> 
> This discussion is lame. There are four possible approaches to this
> problem:
> (1) Discontinue UCB. Inacceptible as it introduces far more issues than
> it is worth.
> (2) Zero the page at random times (e.g. scheduling). Doesn't fix the
> issue, just makes the window smaller.
> (3) Trap on write. Horrible performance, MP issues.
> (4) Disallow mmap for write on not page-sized files. Standard compliance
> issues.

5) Zero the page at mmap time.  Fixes all cases except two concurrently
running processes reading and writing to the area beyond the end of the
file.  Some performance drop, but less than the other options.

eric