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

Linus Torvalds
2008-09-25T15:42:13Z

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.

So all cpumask functions should always return pointers and/or take
pointers to be filled in. That's true *regardless* of how we actually are
to then allocate them.

So forget returning cpumasks. It's irrelevant.

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_.

So all your arguments are invalid, because you're looking at the wrong
thing. The thing that I was talking about is converting current code that
has

random_function(..)
{
cpumask_t mask;

.. do something with mask ...
}

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.

Linus