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

From: Michael S. Tsirkin
Date: Wed Jun 17 2009 - 10:46:30 EST

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.

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