Re: [PATCH 08/10] NOTIFIER: Take over TIF_MCE_NOTIFY and implement task return notifier

[Borislav Petkov <bp@amd64.org>]

On Mon, Jun 13, 2011 at 08:47:05AM -0400, Avi Kivity wrote:
> > HOWEVER, AFAICT, if the page is mapped multiple times,
> > killing/recovering the current task doesn't help from another core
> > touching it and causing a follow-up MCE. So holding off all the cores
> > from scheduling userspace in some manner might be the superior solution.
> > Especially if you don't execute the #MC handler on all CPUs as is the
> > case on AMD.
> >
> 
> That's basically impossible, since the other cores may be in fact 
> executing userspace, with the next instruction accessing the bad page.  
> In fact the access may have been started simultaneously with the one 
> that triggered the #MC.

True.

> The best you can do is IPI everyone as soon as you've caught the #MC,
> but you have to be prepared for multiple #MC for the same page. Once
> you have that, global synchronization is not so important anymore.

Yeah, in the multiple #MC case the memory_failure() thing should
probably be made reentrant-safe (if it is not yet). And in that case,
we'll be starting a worker thread on each CPU that caused an MCE from
accessing that page. The thread that manages to clear all the mappings
of our page simply does so while the others should be able to 'see' that
there's no work to be done anymore (PFN is not mapped in the pagetables
anymore) and exit without doing anything. Yeah, sounds doable with the
irq_work_queue -> user_return_notifier flow.

Thanks.

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Boris.

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