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    SubjectRe: [Bug #11342] Linux 2.6.27-rc3: kernel BUG at mm/vmalloc.c - bisected
    Ingo Molnar wrote:
    > * David Miller <> wrote:
    >> From: Ingo Molnar <>
    >> 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.

    Iirc, it was the problem of basing percpu variables at zero that hit
    problems with various gcc toolset versions. I don't remember any
    version problems with cpumask's on the stack, they all failed the
    same way... :-)
    > 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.

    This is a great idea!
    > 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

    Removing cpumask_t's from the stack is fairly straight forward. The
    problem of changing all functions to expect a cpumask pointer via a
    global change is much more problematic. And of course all those
    functions that return a cpumask value would need to be addressed.


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