Re: [PATCH][v2] fork_init: fix division by zero

Andrew Morton
2008-12-12T01:08:39Z

On Fri, 12 Dec 2008 11:48:29 +1100 Paul Mackerras <paulus@samba.org> wrote:

> Andrew Morton writes:
>
> > > +#if (8 * THREAD_SIZE) > PAGE_SIZE
> > > max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> > > +#else
> > > + max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
> > > +#endif
> >
> > The expression you've chosen here can be quite inacccurate, because
> > ((PAGE_SIZE / (8 * THREAD_SIZE)) is a small number. The way to
> > preserve accuracy is
>
> The assumption is that THREAD_SIZE is a power of 2, as is PAGE_SIZE.
>
> I think Yuri should be increasing THREAD_SIZE for the larger page
> sizes he's implementing, because we have on-stack arrays whose size
> depends on the page size. I suspect that having THREAD_SIZE less than
> 1/8 of PAGE_SIZE risks stack overflows, and the better fix is for Yuri
> to make sure THREAD_SIZE is at least 1/8 of PAGE_SIZE. (In fact, more
> may be needed - someone should work out what fraction is actually
> needed.)

OK, yes.

It's the MAX_BUF_PER_PAGE arrays which will hurt. iirc they nest
three-deep on some codepaths.