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

    * Rusty Russell <rusty@rustcorp.com.au> wrote:

    > I can't see a neater way down this path, and I don't want to lose
    > const.
    >
    > I can see three alternatives:
    > 1) An ONSTACK_CPUMASK(name) macro which declares "struct cpumask name[1]" or
    > "struct cpumask *name". Same idea as yours, without the typedef.
    > 2) Use a normal struct for cpumask, make everyone use pointers, but have an
    > struct cpumask *alloc_stack_cpumask() which uses alloca() for small
    > NR_CPUS.
    > 3) Same, but just use kmalloc everywhere. Optimize important cases by hand.
    >
    > Anyone see a better way?

    since most of the important cpumasks in high-perf codepaths are already
    pre-constructed and embedded in some existing object (say task_struct),
    i think a variant of #3 is the best approach:

    - get rid of cpumask_t and use 'struct cpumask' everywhere.

    - do not expose normal kernel code to struct cpumask's definition, only
    declare the type via 'struct cpumask;' in sched.h - a'la
    kmem_cache_t.

    - even hide the structure from sched.h - use an extra indirection for
    struct cpumask *cpus_allowed in task_struct and be done with it.

    - have normal cpumask object alloc/free codepaths.

    - optimize any remaining important cases by hand, if needed. (the
    scheduler mostly)

    (wrt. #2, alloca() is a nightmare i think.)

    Ingo


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