Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

Michael S. Tsirkin
2009-06-17T14:46:26Z

On Tue, Jun 16, 2009 at 02:09:38PM -0400, Gregory Haskins wrote:
> > What do you mean by copy_to_user(other->mm) here? If you are going to switch
> > to another mm, then I think current->mm must be valid (I think it's not enough
> > that you can sleep). So preemptible() might not be enough.
> >
>
> I dont currently use switch_mm, if that is what you mean. I save the
> task_struct into the appropriate context so current->mm doesn't matter
> to me. I never use it. All I need (afaik) is to acquire the proper
> mutex first. I am not an MM expert, so perhaps I have this wrong but it
> does appear to work properly even from kthread context.
>
> -Greg
>
>

I think I saw get_user_pages + memcpy in your patch. Yes, that works
without switching contexts but it's slower than copy to user if you are
in the right context, and AFAIK it's slower than get_user_pages_fast.

--
MST