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

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Date:	Fri, 25 Jul 2014 23:57:18 +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 02:42:13PM -0700, Jesse Barnes wrote:
> On Fri, 25 Jul 2014 23:38:06 +0200
> Joerg Roedel <joro@...tes.org> wrote:
> > 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.
> 
> Making the _end callback just do another TLB flush is fine too, but it
> would be nice to have the consistency of (1).  I can live with either
> though, as long as the callbacks are well documented.

You are right, having this consistency would be good. The more I think
about it, the more it makes sense to go with option 2). Option 1) would
mean that invalidate_range is explicitly called right before
invalidate_range_end at some places. Doing this implicitly like in
option 2) is cleaner and less error-prone. And the list of mmu_notifiers
needs only be traversed once in invalidate_range_end(), so additional
overhead is minimal. I'll update patch 3 for this, unless there are
other opinions.


	Joerg

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