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    SubjectRe: frequent lockups in 3.18rc4
    On Fri, Nov 21, 2014 at 11:25:06AM -0500, Tejun Heo wrote:
    >
    > * Static percpu areas wouldn't trigger fault lazily. Note that this
    > is not necessarily because the first percpu chunk which contains the
    > static area is embedded inside the kernel linear mapping. Depending
    > on the memory layout and boot param, percpu allocator may choose to
    > map the first chunk in vmalloc space too; however, this still works
    > out fine because at that point there are no other page tables and
    > the PUD entries covering the first chunk is faulted in before other
    > pages tables are copied from the kernel one.

    That sounds correct.

    >
    > * NMI used to be a problem because vmalloc fault handler couldn't
    > safely nest inside NMI handler but this has been fixed since and it
    > should work fine from NMI handlers now.

    Right. Of course "should work fine" does not excatly mean "will work fine".


    >
    > * Function tracers are problematic because they may end up nesting
    > inside themselves through triggering a vmalloc fault while accessing
    > dynamic percpu memory area. This may lead to recursive locking and
    > other surprises.

    The function tracer infrastructure now has a recursive check that happens
    rather early in the call. Unless the registered OPS specifically states
    it handles recursions (FTRACE_OPS_FL_RECUSION_SAFE), ftrace will add the
    necessary recursion checks. If a registered OPS lies about being recusion
    safe, well we can't stop suicide.

    Looking at kernel/trace/trace_functions.c: function_trace_call() which is
    registered with RECURSION_SAFE, I see that the recursion check is done
    before the per_cpu_ptr() call to the dynamically allocated per_cpu data.

    It looks OK, but...

    Oh! but if we trace the page fault handler, and we fault here too
    we just nuked the cr2 register. Not good.

    -- Steve


    >
    > Are there other cases where the lazy vmalloc faults can break things?


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