linux-kernel - Re: [RFC][PATCH 2/6] pagewalk: only split huge pages when necessary

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Date:	Tue, 01 Feb 2011 07:03:56 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael J Wolf <mjwolf@...ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC][PATCH 2/6] pagewalk: only split huge pages when necessary

On Tue, 2011-02-01 at 11:04 +0100, Johannes Weiner wrote:
> On Mon, Jan 31, 2011 at 04:33:59PM -0800, Dave Hansen wrote:
> > Right now, if a mm_walk has either ->pte_entry or ->pmd_entry
> > set, it will unconditionally split and transparent huge pages
> > it runs in to.  In practice, that means that anyone doing a
> > 
> >       cat /proc/$pid/smaps
> > 
> > will unconditionally break down every huge page in the process
> > and depend on khugepaged to re-collapse it later.  This is
> > fairly suboptimal.
> > 
> > This patch changes that behavior.  It teaches each ->pmd_entry
> > handler (there are three) that they must break down the THPs
> > themselves.  Also, the _generic_ code will never break down
> > a THP unless a ->pte_entry handler is actually set.
> > 
> > This means that the ->pmd_entry handlers can now choose to
> > deal with THPs without breaking them down.
> 
> Makes perfect sense.  But you forgot to push down the splitting into
> the two handlers in mm/memcontrol.c. 

I did indeed.  I'll go fix those up.  Thanks for the review!

-- Dave

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