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

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Date:	Fri, 05 Apr 2013 16:54:44 +0800
From:	Simon Jeons <simon.jeons@...il.com>
To:	Michal Hocko <mhocko@...e.cz>
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

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".

>

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