Date: Wed, 17 Jun 2009 17:45:40 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Gregory Haskins <ghaskins@novell.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, avi@redhat.com,
       davidel@xmailserver.org, paulmck@linux.vnet.ibm.com, mingo@elte.hu
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based
	notifier interface
Message-ID: <20090617144540.GB3089@redhat.com>
References: <20090616140240.GA9401@redhat.com> <4A37A7FC.4090403@novell.com> <20090616143816.GA18196@redhat.com> <4A37B0BB.3020005@novell.com> <20090616145502.GA1102@redhat.com> <4A37B832.6040206@novell.com> <20090616154150.GA17494@redhat.com> <4A37C592.2030407@novell.com> <20090616175300.GA24514@redhat.com> <4A37DFE2.2020506@novell.com>
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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
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