linux-kernel - Re: [patch 1/3] mmu

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Date:	Thu, 31 Jan 2008 21:01:05 -0600
From:	Jack Steiner <steiner@....com>
To:	Christoph Lameter <clameter@....com>
Cc:	Robin Holt <holt@....com>, Andrea Arcangeli <andrea@...ranet.com>,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [patch 1/3] mmu_notifier: Core code

On Thu, Jan 31, 2008 at 06:39:19PM -0800, Christoph Lameter wrote:
> On Thu, 31 Jan 2008, Robin Holt wrote:
> 
> > Jack has repeatedly pointed out needing an unregister outside the
> > mmap_sem.  I still don't see the benefit to not having the lock in the mm.
> 
> I never understood why this would be needed. ->release removes the 
> mmu_notifier right now.

Christoph -

We discussed this earlier this week. Here is part of the mail:

------------

> > There currently is no __mmu_notifier_unregister(). Oversite???
>
> No need. mmu_notifier_release implies an unregister and I think that is
> the most favored way to release resources since it deals with the RCU
> quiescent period.


I currently unlink the mmu_notifier when the last GRU mapping is closed. For
example, if a user does a:

        gru_create_context();
        ...
        gru_destroy_context();

the mmu_notifier is unlinked and all task tables allocated
by the driver are freed. Are you suggesting that I leave tables
allocated until the task terminates??

Why is that better? What problem do I cause by trying
to free tables as soon as they are not needed?


-----------------------------------------------

> Christoph responded:
> > the mmu_notifier is unlinked and all task tables allocated
> > by the driver are freed. Are you suggesting that I leave tables
> > allocated until the task terminates??
>
> You need to leave the mmu_notifier structure allocated until the next
> quiescent rcu period unless you use the release notifier.

I assumed that I would need to use call_rcu() or synchronize_rcu()
before the table is actually freed. That's still on my TODO list.



--- jack
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