linux-kernel - Re: [PATCH 01/18] mm, hugetlb: protect reserved pages when softofflining requests the pages

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Message-ID: <CAJd=RBCj_wAHjv10FhhX+Fzx8p4ybeGykEfvqF=jZaok3s+j9w@mail.gmail.com>
Date:	Wed, 31 Jul 2013 23:25:04 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	David Gibson <david@...son.dropbear.id.au>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH 01/18] mm, hugetlb: protect reserved pages when
 softofflining requests the pages

On Wed, Jul 31, 2013 at 2:37 PM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
> On Wed, Jul 31, 2013 at 02:21:38PM +0800, Hillf Danton wrote:
>> On Wed, Jul 31, 2013 at 12:41 PM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
>> > On Wed, Jul 31, 2013 at 10:49:24AM +0800, Hillf Danton wrote:
>> >> On Wed, Jul 31, 2013 at 10:27 AM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
>> >> > On Mon, Jul 29, 2013 at 03:24:46PM +0800, Hillf Danton wrote:
>> >> >> On Mon, Jul 29, 2013 at 1:31 PM, Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
>> >> >> > alloc_huge_page_node() use dequeue_huge_page_node() without
>> >> >> > any validation check, so it can steal reserved page unconditionally.
>> >> >>
>> >> >> Well, why is it illegal to use reserved page here?
>> >> >
>> >> > If we use reserved page here, other processes which are promised to use
>> >> > enough hugepages cannot get enough hugepages and can die. This is
>> >> > unexpected result to them.
>> >> >
>> >> But, how do you determine that a huge page is requested by a process
>> >> that is not allowed to use reserved pages?
>> >
>> > Reserved page is just one for each address or file offset. If we need to
>> > move this page, this means that it already use it's own reserved page, this
>> > page is it. So we should not use other reserved page for moving this page.
>> >
>> Hm, how do you determine "this page" is not buddy?
>
> If this page comes from the buddy, it doesn't matter. It imply that
> this mapping cannot use reserved page pool, because we always allocate
> a page from reserved page pool first.
>
A buddy page also implies, if the mapping can use reserved pages, that no
reserved page was available when requested. Now we can try reserved
page again.
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