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    SubjectRe: [RFC PATCH] x86: Barf when faults happen in NMI
    On Tue, Sep 28, 2010 at 12:36:18PM +0200, Ingo Molnar wrote:
    >
    > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
    >
    > > In x86, faults exit by executing the iret instruction, which then
    > > reenables NMIs if we faulted in NMI context. Then if a fault happens
    > > in NMI, another NMI can nest after the fault exits.
    > >
    > > But we don't yet support nested NMIs because we have only one NMI
    > > stack. To prevent that, trigger a bug when a fault happens in NMI
    > > context.
    > >
    > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > > Cc: Ingo Molnar <mingo@elte.hu>
    > > Cc: Thomas Gleixner <tglx@linutronix.de>
    > > Cc: H. Peter Anvin <hpa@zytor.com>
    > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    > > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > > ---
    > >
    > > I first thought about putting it in the vmalloc fault path only. But
    > > then I saw more occasions for the kernel to fault (kmemcheck or so),
    > > and so I thought it should be better put in the all in one path. But I
    > > suspect you won't like that conditional in the big x86 fault path.
    > >
    > >
    > > arch/x86/mm/fault.c | 2 ++
    > > 1 files changed, 2 insertions(+), 0 deletions(-)
    > >
    > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
    > > index 4c4508e..80c997e 100644
    > > --- a/arch/x86/mm/fault.c
    > > +++ b/arch/x86/mm/fault.c
    > > @@ -955,6 +955,8 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
    > > int write;
    > > int fault;
    > >
    > > + BUG_ON(in_nmi());
    > > +
    >
    > Hm, this is a fastpath, so it would be nice to put this into the vmalloc
    > and kmemcheck paths (even though it's less clean that way).
    >
    > Also, a WARN_ON_ONCE() would probably help more in getting bug reports
    > out of such systems, than a BUG_ON() crashed kernel.
    >
    > Thanks,
    >
    > Ingo


    Ok, will resend then.

    Thanks.



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