Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

Rusty Russell
2008-09-26T05:25:29Z

On Friday 26 September 2008 01:42:13 Linus Torvalds wrote:
> On Thu, 25 Sep 2008, Rusty Russell wrote:
> > This turns out to be awful in practice, mainly due to const.
> > Consider:
> >
> > #ifdef CONFIG_CPUMASK_OFFSTACK
> > typedef unsigned long *cpumask_t;
> > #else
> > typedef unsigned long cpumask_t[1];
> > #endif
> >
> > cpumask_t returns_cpumask(void);
>
> No. That's already broken. You cannot return a cpumask_t, regardless of
> interface. We must not do it regardless of how we pass those things
> around, since it generates _yet_ another temporary on the stack for the
> return slot for any kind of structure.

No, for large NR_CPUS, cpumask_t is a pointer as shown. And we have numerous
basic functions which return a cpumask_t. Yes, this is part of the problem.

> What _is_ relevant is how we allocate them when we need temporary CPU
> masks. And _that_ is where my suggestion comes in. For small NR_CPUS, we
> really do want to allocate them on the stack, because calling kmalloc for
> a 4- or 8-byte allocation is just _stupid_.

Right, but cpumask_t is used for far more than stack decls, thus the problems.

I can make a separate "cpumask_stack_t" and use your method tho. I think that
might even reduce churn and allow us to do this in parts.

> which has to be converted some way. And I think it needs to be converted
> in a way that does *not* force us to call kmalloc() for idiotically small
> values.

Yeah, got that. But your suggestion to change cpumask_t turned out horribly
ugly.

Cheers,
Rusty.