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

Paul Mackerras
2008-12-12T00:49:40Z

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.)

Paul.