Re: notify_page_fault() problem

[Andi Kleen <andi@firstfloor.org>]

> However, vmalloc_sync_all() is i386 and x86_64 specific as well
> as their change to register_page_fault_notifier().  I don't see
> other platform doing anything else doing anything special in their
> register_page_fault_notifier().  

They probably just haven't tested this particular case yet.
x86 also did it originally to handle NMI notifiers, which is a x86 special
(nested pagefault in NMI can lead to stack corruption because
NMIs are only blocked until the next IRET)

> I have trouble believing that x86
> and ARM are unique somehow with needing to address this problem.
> Why doesn't anyone else hit this?  Is it a lurking problem or are
> there other fixes in other forms out there?

The standard kprobes notifier is not modular so it won't hit this.
 
> I guess part of the answer has to do with what people's expectations
> are for intercepting faults with their kprobes fault handler though.

Yes, some have pretty broad exceptions.  It might be possible
to move it to a kernel address only path, but then some debuggers
seem to want to debug user mode too.

But you're right there has been grumbling about the overhead
of the notifier call in the hot path.

-Andi