linux-kernel - Re: [PATCH 1/3] mmu_notifier: Add mmu_notifier_invalidate

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Message-ID: <20140725213806.GN14017@8bytes.org>
Date:	Fri, 25 Jul 2014 23:38:06 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Jerome Glisse <jglisse@...hat.com>, jroedel@...e.de,
	Jay.Cornwall@....com, Oded.Gabbay@....com, John.Bridgman@....com,
	Suravee.Suthikulpanit@....com, ben.sander@....com,
	David Woodhouse <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 1/3] mmu_notifier: Add mmu_notifier_invalidate_range()

On Fri, Jul 25, 2014 at 01:16:39PM -0700, Jesse Barnes wrote:
> > To allow managing external TLBs the MMU-notifiers need to
> > catch the moment when pages are unmapped but not yet freed.
> > This new notifier catches that moment and notifies the
> > interested subsytem when pages that were unmapped are about
> > to be freed. The new notifier will only be called between
> > invalidate_range_start()/end().
> 
> So if we were actually sharing page tables, we should be able to make
> start/end no-ops and just use this new callback, assuming we didn't
> need to do any other serialization or debug stuff, right?

Well, not completly. What you need with this patch-set is a
invalidate_range and an invalidate_end call-back. There are call sites
of the start/end functions where the TLB flush happens after the _end
notifier (or at least can wait until _end is called). I did not add
invalidate_range calls to these places (yet). But you can easily discard
invalidate_range_start, any flush done in there is useless with shared
page-tables.

I though about removing the need for invalidate_range_end too when
writing the patches, and possible solutions are

	1) Add mmu_notifier_invalidate_range() to all places where
	   start/end is called too. This might add some unnecessary
	   overhead.

	2) Call the invalidate_range() call-back from the
	   mmu_notifier_invalidate_range_end too.

	3) Just let the user register the same function for
	   invalidate_range and invalidate_range_end

I though that option 1) adds overhead that is not needed (but it might
not be too bad, the overhead is an additional iteration over the
mmu_notifer list when there are no call-backs registered).

Option 2) might also be overhead if a user registers different functions
for invalidate_range() and invalidate_range_end(). In the end I came to
the conclusion that option 3) is the best one from an overhead POV.

But probably targeting better usability with one of the other options is
a better choice? I am open for thoughts and suggestions on that.

	Joerg

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