linux-kernel - Re: [patch 1/4] mmu

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Message-ID: <20080126115639.GQ26420@sgi.com>
Date:	Sat, 26 Jan 2008 05:56:39 -0600
From:	Robin Holt <holt@....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>,
	Nick Piggin <npiggin@...e.de>, kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 1/4] mmu_notifier: Core code

> > > 1. invalidate_all()
> > 
> > That will be fine as long as we can unregister the ops notifier and free
> > the structure.  Otherwise, we end up being called needlessly.
> 
> No you cannot do that because there are still callbacks that come later. 
> The invalidate_all may lead to invalidate_range() doing nothing for this 
> mm. The ops notifier and the freeing of the structure has to wait until 
> release().

Could you be a little more clear here?  If you are saying that the other
callbacks will need to do work?  I can assure you we will clean up those
pages and raise memory protections.  It will also be done in a much more
efficient fashion than the individual callouts.

If, on the other hand, you are saying we can not because of the way
we traverse the list, can we return a result indicating to the caller
we would like to be unregistered and then the mmu_notifier code do the
remove followed by a call to the release notifier?

> 
> > > 2. invalidate_range() for each vma
> > > 
> > > 3. release()
> > > 
> > > We cannot simply move the call up because there will be future range 
> > > callbacks on vma invalidation.
> > 
> > I am not sure what this means.  Right now, if you were to notify XPMEM
> > the process is exiting, we would take care of all the recalling of pages
> > exported by this process, clearing those pages cache lines from cache,
> > and raising memory protections.  I would assume that moving the callout
> > earlier would expect the same of every driver.
> 
> That does not sync with the current scheme of the invalidate_range() 
> hooks. We would have to do a global invalidate early and then place the 
> other invalidate_range hooks in such a way that none is called in later in 
> process exit handling.

But if the notifier is removed from the list following the invalidate_all
callout, there would be no additional callouts.

Thanks,
Robin
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