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

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Date:	Tue, 26 Aug 2008 10:08:22 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"David Miller" <davem@...emloft.net>, Alan.Brunelle@...com,
	travis@....com, tglx@...utronix.de, rjw@...k.pl,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	kernel-testers@...r.kernel.org,
	"Andrew Morton" <akpm@...ux-foundation.org>, arjan@...ux.intel.com,
	rusty@...tcorp.com.au
Subject: Re: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected

On Tue, Aug 26, 2008 at 9:51 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>
> 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!
>

that is good for local variables.

for global variables, need to allocate them in some point. may need one
int cpumask_size;

cpumask_t online_cpu_map;
DEFINE_DYN_ARRAY(online_cpu_map, sizeof(unsigned long), cpumask_size,
PAGE_SIZE, NULL);

or something like that.

YH
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