linux-kernel - Re: [PATCH 0/6] mm/hugetlb: gigantic hugetlb page pools shrink supporting

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Date:	Fri, 5 Apr 2013 11:52:58 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Simon Jeons <simon.jeons@...il.com>
Cc:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Hillf Danton <dhillf@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] mm/hugetlb: gigantic hugetlb page pools shrink
 supporting

On Fri 05-04-13 16:54:44, Simon Jeons wrote:
> Hi Michal,
> On 04/05/2013 04:12 PM, Michal Hocko wrote:
> >On Fri 05-04-13 07:41:23, Wanpeng Li wrote:
> >>On Thu, Apr 04, 2013 at 06:17:46PM +0200, Michal Hocko wrote:
> >>>On Thu 04-04-13 17:09:08, Wanpeng Li wrote:
> >>>>order >= MAX_ORDER pages are only allocated at boot stage using the
> >>>>bootmem allocator with the "hugepages=xxx" option. These pages are never
> >>>>free after boot by default since it would be a one-way street(>= MAX_ORDER
> >>>>pages cannot be allocated later), but if administrator confirm not to
> >>>>use these gigantic pages any more, these pinned pages will waste memory
> >>>>since other users can't grab free pages from gigantic hugetlb pool even
> >>>>if OOM, it's not flexible.  The patchset add hugetlb gigantic page pools
> >>>>shrink supporting. Administrator can enable knob exported in sysctl to
> >>>>permit to shrink gigantic hugetlb pool.
> >>>I am not sure I see why the new knob is needed.
> >>>/sys/kernel/mm/hugepages/hugepages-*/nr_hugepages is root interface so
> >>>an additional step to allow writing to the file doesn't make much sense
> >>>to me to be honest.
> >>>
> >>>Support for shrinking gigantic huge pages makes some sense to me but I
> >>>would be interested in the real world example. GB pages are usually used
> >>>in very specific environments where the amount is usually well known.
> >>Gigantic huge pages in hugetlb means h->order >= MAX_ORDER instead of GB
> >>pages. ;-)
> >Yes, I am aware of that but the question remains the same (and
> >unanswered). What is the use case?
> 
> As patch description, "if administrator confirm not to use these
> gigantic pages any more, these pinned pages will waste memory since
> other users can't grab free pages from gigantic hugetlb pool even if
> OOM".

Is this a use case that we care about? How often something like that
happens? I understand this is "nice to have" but I am interested whether
somebody actually _needs_ this.
-- 
Michal Hocko
SUSE Labs
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