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    SubjectRe: [PATCH 3/4] kernel/fork: switch vmapped stack callation to __vmalloc_area()
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    On 21.02.2018 03:16, Andrew Morton wrote:
    > On Tue, 23 Jan 2018 16:57:21 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:
    >
    >> # stress-ng --clone 100 -t 10s --metrics-brief
    >> at 32-core machine shows boost 35000 -> 36000 bogo ops
    >>
    >> Patch 4/4 is a kind of RFC.
    >> Actually per-cpu cache of preallocated stacks works faster than buddy allocator thus
    >> performance boots for it happens only at completely insane rate of clones.
    >>
    >
    > I'm not really sure what to make of this patchset. Is it useful in any
    > known real-world use cases?

    Not yet. Feel free to ignore last patch.

    >
    >> + This option neutralize stack overflow protection but allows to
    >> + achieve best performance for syscalls fork() and clone().
    >
    > That sounds problematic, but perhaps acceptable if the fallback only
    > happens rarely.
    >
    > Can this code be folded into CONFIG_VMAP_STACk in some cleaner fashion?
    > We now have options for non-vmapped stacks, vmapped stacks and a mix
    > of both.
    >
    > And what about this comment in arch/Kconfig:VMAP_STACK:
    >
    > This is presently incompatible with KASAN because KASAN expects
    > the stack to map directly to the KASAN shadow map using a formula
    > that is incorrect if the stack is in vmalloc space.
    >
    >
    > So VMAP_STACK_AS_FALLBACK will intermittently break KASAN?
    >

    All of this (including CONFIG_VMAP_STACK) could be turned into boot option.
    I think this would be a best solution.

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