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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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