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SubjectRe: [PATCH] x86: Remove WARN_ON(in_nmi()) from vmalloc_fault

* Steven Rostedt <rostedt@goodmis.org> wrote:

> On Wed, 16 Oct 2013 08:11:18 +0200
> Ingo Molnar <mingo@kernel.org> wrote:
>
> >
> > * Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> > > Since the NMI iretq nesting has been fixed, there's no reason that
> > > an NMI handler can not take a page fault for vmalloc'd code. No locks
> > > are taken in that code path, and the software now handles nested NMIs
> > > when the fault re-enables NMIs on iretq.
> > >
> > > Not only that, if the vmalloc_fault() WARN_ON_ONCE() is hit, and that
> > > warn on triggers a vmalloc fault for some reason, then we can go into
> > > an infinite loop (the WARN_ON_ONCE() does the WARN() before updating
> > > the variable to make it happen "once").
> > >
> > > Reported-by: "Liu, Chuansheng" <chuansheng.liu@intel.com>
> > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> >
> > Would be nice to see the warning quoted that triggered this.
>
> Sure, want me to add this to the change log?

Yeah, that would be helpful - but only the stack trace portion I suspect,
to make it clear what caused the fault.

The one posted in the thread shows:

[ 17.148755] [<c2825b08>] do_page_fault+0x8/0x10
[ 17.153926] [<c2823066>] error_code+0x5a/0x60
[ 17.158905] [<c2825b00>] ? __do_page_fault+0x4a0/0x4a0
[ 17.164760] [<c208d1a9>] ? module_address_lookup+0x29/0xb0
[ 17.170999] [<c208dddb>] kallsyms_lookup+0x9b/0xb0
[ 17.186804] [<c208def4>] sprint_symbol+0x14/0x20
[ 17.192063] [<c208df1e>] __print_symbol+0x1e/0x40
[ 17.197430] [<c25e00d7>] ? ashmem_shrink+0x77/0xf0
[ 17.202895] [<c25e13e0>] ? logger_aio_write+0x230/0x230
[ 17.208845] [<c205bdf5>] ? up+0x25/0x40
[ 17.213242] [<c2039cb7>] ? console_unlock+0x337/0x440
[ 17.218998] [<c2818236>] ? printk+0x38/0x3a
[ 17.223782] [<c20006d0>] __show_regs+0x70/0x190
[ 17.228954] [<c200353a>] show_regs+0x3a/0x1b0
[ 17.233931] [<c2818236>] ? printk+0x38/0x3a
[ 17.238717] [<c2824182>] arch_trigger_all_cpu_backtrace_handler+0x62/0x80
[ 17.246413] [<c2823919>] nmi_handle.isra.0+0x39/0x60
[ 17.252071] [<c2823a29>] do_nmi+0xe9/0x3f0

So kallsyms_lookup() faulted, while the NMI watchdog triggered a
show_regs()? How is that possible?

Thanks,

Ingo


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