Re: Panic and page fault in loop during handling NMI backtracehandler

[Frederic Weisbecker]

On Tue, Oct 15, 2013 at 02:37:17PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 15, 2013 at 02:18:53PM +0200, Frederic Weisbecker wrote:
> > On Tue, Oct 15, 2013 at 02:01:04AM +0000, Liu, Chuansheng wrote:
> > > We meet one issue that during trigger all CPU backtrace, but during in the NMI handler arch_trigger_all_cpu_backtrace_handler,
> > > It hit the PAGE fault, then PAGE fault is in loop, at last the thread stack overflow, and system panic.
> > > 
> > > Anyone can give some help? Thanks.
> > 
> > Looks like we re-enter the fault several times. On x86-32, NMIs can
> > fault if they dereference vmalloc'ed area. I wonder if the module thing
> > we lookup in the NMI is stored on some vmalloc'ed area.
> 
> IIRC modules are indeed allocated using vmalloc. See module_alloc()
> using vmalloc_exec()

Right. At least the module text. Now I'm not sure the module_address_lookup()
dereference that. But there many other objects allocated in module.c that use
alloc_percpu(), which in turn can use vmalloc.

IIRC Steve made the NMIs safely faultable. So may be we can remove the WARN_ON in
do_page_fault(). It may not be a good idea to allow fault in NMIs though. Steve?
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