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

[Linus Torvalds]

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
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