linux-kernel - Re: [PATCH 1/5] pagewalk: only split huge pages when necessary

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Message-ID: <alpine.DEB.2.00.1102221316400.5929@chino.kir.corp.google.com>
Date:	Tue, 22 Feb 2011 13:21:48 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
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>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH 1/5] pagewalk: only split huge pages when necessary

On Mon, 21 Feb 2011, Dave Hansen wrote:

> 
> v2 - rework if() block, and remove  now redundant split_huge_page()
> 
> Right now, if a mm_walk has either ->pte_entry or ->pmd_entry
> set, it will unconditionally split any 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 five) 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.
> 
> Acked-by: Mel Gorman <mel@....ul.ie>
> Signed-off-by: Dave Hansen <dave@...ux.vnet.ibm.com>

Acked-by: David Rientjes <rientjes@...gle.com>

Thanks for adding the comment about ->pmd_entry() being required to split 
the pages in include/linux/mm.h!
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