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

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Message-ID: <20110201100433.GH19534@cmpxchg.org>
Date:	Tue, 1 Feb 2011 11:04:34 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
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 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.
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