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

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Date:	Sun, 4 Aug 2013 13:10:05 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	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 Fri, Aug 2, 2013 at 12:17 AM, Aneesh Kumar K.V
<aneesh.kumar@...ux.vnet.ibm.com> wrote:
> Hillf Danton <dhillf@...il.com> writes:
>
>> 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.
>
> I didn't quiet get that. My understanding is, the new page we are

Neither did I ;)

> allocating here for soft offline should not be allocated from the
> reserve pool. If we do that we may possibly have allocation failure
> later for a request that had done page reservation. Now to
> avoid that we make sure we have enough free pages outside reserve pool
> so that we can dequeue the huge page. If not we use buddy to allocate
> the hugepage.
>
What if it is a mapping with HPAGE_RESV_OWNER set?
Or can we block owner from using any page available here?
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