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

    * David Miller <davem@davemloft.net> wrote:

    > From: Ingo Molnar <mingo@elte.hu>
    > Date: Tue, 26 Aug 2008 09:22:20 +0200
    >
    > > And i guess the next generation of 4K CPUs support should just get away
    > > from cpumask_t-on-kernel-stack model altogether, as the current model is
    > > not maintainable. We tried the on-kernel-stack variant, and it really
    > > does not work reliably. We can fix this in v2.6.28.
    >
    > I recenetly did some work on sparc64 to use cpumask pointers as much
    > as possible.
    >
    > The only case that didn't work was due to a limitation in arch
    > interfaces for the new generic smp_call_function() code. It passes a
    > cpumask_t instead of a pointer to one via
    > arch_send_call_function_ipi().
    >
    > But other than that, the whole sparc64 SMP stuff uses cpumask_t
    > pointers only.

    nice!

    > What it comes down to is that you have to do the "self cpu" and other
    > tests in the cross-call dispatch routines themselves, instead of at
    > the top-level working on cpumask_t objects.
    >
    > Otherwise you have to modify cpumask_t objects and thus pluck them
    > onto the stack where they take up silly amounts of space.

    What we did was this: we added MAXSMP which just revs up all the SMP
    tunables to the maximum, so that we can see any problems early in
    testing.

    And we triggered problems, and we fixed a couple of regressions all
    around stack footprint. But we didnt catch all of them - some were gcc
    version dependent and configuration dependent. So i think it's safe to
    say that the whole concept of allowing such a large cpumask_t to be on
    the stack is fragile.

    Hence, i think the best way forward is to change the whole cpumask_t
    concept and disallow explicit masks altogether. It's so easy to smack a
    cpumask_t variable on the stack and nothing really warns about it, and
    any function can become part of a nested call sequence.

    So i think the dynamics of it has to be changed: we need a get/put API
    and we need to make on-stack cpumask illegal on the build level (in
    generic code at least). This has been Rusty's main argument early on i
    think, and i now concur.

    Ingo


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