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

[Linus Torvalds]

On Tue, 26 Aug 2008, Yinghai Lu wrote:
> 
> wonder if could use "unsigned long *" directly.

I would actually suggest something like this:

 - we continue to have a magic "cpumask_t".

 - we do different cases for big and small NR_CPUS:

	#if NR_CPUS <= BITS_PER_LONG

	/*
	 * Make it an array - that way passing it as an argument will
	 * always pass it as a pointer!
	 */
	typedef unsigned long cpumask_t[1];

	static inline void create_cpumask(cpumask_t *p)
	{
		*p = 0;
	}
	static inline void free_cpumask(cpumask_t *p)
	{
	}

	#else

	typedef unsigned long *cpumask_t;

	static inline void create_cpumask(cpumask_t *p)
	{
		*p = kcalloc(..);
	}

	static inline void free_cpumask(cpumask_t *p)
	{
		kfree(*p);
	}

	#endif

and now after you do this, you can just do something like

	cpumask_t mycpu;

	create_cpumask(&mycpu);
	..
	free_cpumask(&mycpu);

and in between, you can use 'cpumask' as a pointer, because even when it 
is an array directly allocated on the stack, the array can always 
degenerate into a pointer by C type rules!

And for the small-NR_CPUS case there is zero overhead.

			Linus
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