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


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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