linux-kernel - Re: [PATCH 01 of 12] Core of mmu notifiers

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Date:	Wed, 23 Apr 2008 09:47:47 -0500
From:	Robin Holt <holt@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	Robin Holt <holt@....com>, Christoph Lameter <clameter@....com>,
	Nick Piggin <npiggin@...e.de>, Jack Steiner <steiner@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, general@...ts.openfabrics.org,
	Hugh Dickins <hugh@...itas.com>, akpm@...ux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 01 of 12] Core of mmu notifiers

On Wed, Apr 23, 2008 at 03:36:19PM +0200, Andrea Arcangeli wrote:
> On Tue, Apr 22, 2008 at 06:07:27PM -0500, Robin Holt wrote:
> > > The only other change I did has been to move mmu_notifier_unregister
> > > at the end of the patchset after getting more questions about its
> > > reliability and I documented a bit the rmmod requirements for
> > > ->release. we'll think later if it makes sense to add it, nobody's
> > > using it anyway.
> > 
> > XPMEM is using it.  GRU will be as well (probably already does).
> 
> XPMEM requires more patches anyway. Note that in previous email you
> told me you weren't using it. I think GRU can work fine on 2.6.26

I said I could test without it.  It is needed for the final version.
It also makes the API consistent.  What you are proposing is equivalent
to having a file you can open but never close.

This whole discussion seems ludicrous.  You could refactor the code to get
the sorted list of locks, pass that list into mm_lock to do the locking,
do the register/unregister, then pass the same list into mm_unlock.

If the allocation fails, you could fall back to the older slower method
of repeatedly scanning the lists and acquiring locks in ascending order.

> without mmu_notifier_unregister, like KVM too. You've simply to unpin
> the module count in ->release. The most important bit is that you've
> to do that anyway in case mmu_notifier_unregister fails (and it can

If you are not going to provide the _unregister callout you need to change
the API so I can scan the list of notifiers to see if my structures are
already registered.

We register our notifier structure at device open time.  If we receive a
_release callout, we mark our structure as unregistered.  At device close
time, if we have not been unregistered, we call _unregister.  If you
take away _unregister, I have an xpmem kernel structure in use _AFTER_
the device is closed with no indication that the process is using it.
In that case, I need to get an extra reference to the module in my device
open method and hold that reference until the _release callout.

Additionally, if the users program reopens the device, I need to scan the
mmu_notifiers list to see if this tasks notifier is already registered.

I view _unregister as essential.  Did I miss something?

Thanks,
Robin
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