linux-kernel - Re: [PATCH 8/8] prepare to remove /proc/sys/vm/hugepages_treat_as

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Date:	Tue, 06 Aug 2013 00:30:03 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, Michal Hocko <mhocko@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH 8/8] prepare to remove
 /proc/sys/vm/hugepages_treat_as_movable

On Tue, Aug 06, 2013 at 07:22:02AM +0530, Aneesh Kumar K.V wrote:
> Naoya Horiguchi <n-horiguchi@...jp.nec.com> writes:
> >> 
> >> Considering that we have architectures that won't support migrating
> >> explicit hugepages with this patch series, is it ok to use
> >> GFP_HIGHUSER_MOVABLE for hugepage allocation ?
> >
> > Originally this parameter was introduced to make hugepage pool on ZONE_MOVABLE.
> > The benefit is that we can extend the hugepage pool more easily,
> > because external fragmentation less likely happens than other zone type
> > by rearranging fragmented pages with page migration/reclaim.
> >
> > So I think using ZONE_MOVABLE for hugepage allocation by default makes sense
> > even on the architectures which don't support hugepage migration.
> 
> But allocating hugepages from ZONE_MOVABLE means we have pages in that
> zone which we can't migrate. Doesn't that impact other features like
> hotplug ?

Memory blocks occupied by hugepages are not removable before this patchset,
whether they are from ZONE_MOVABLE or not, and the hugepage users accepted
it for now. So I think this change doesn't make things worse than now. 

It can be more preferable to switch on/off __GFP_MOVABLE flag depending on
archs without using the tunable parameter. I'm ok for this direction, but
I want to do it as a separate work.

Thanks,
Naoya Horiguchi
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