linux-kernel - Re: [PATCH 09/10] memory-hotplug: enable memory hotplug to handle hugepage

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Date:	Tue, 09 Apr 2013 18:43:36 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Hillf Danton <dhillf@...il.com>, Michal Hocko <mhocko@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/10] memory-hotplug: enable memory hotplug to handle
 hugepage

On Tue, Apr 09, 2013 at 05:27:44PM -0400, KOSAKI Motohiro wrote:
> >> numa_node_id() is really silly. This might lead to allocate from offlining node.
> >
> > Right, it should've been alloc_huge_page().
> >
> >> and, offline_pages() should mark hstate as isolated likes normal pages for prohibiting
> >> new allocation at first.
> >
> > It seems that alloc_migrate_target() calls alloc_page() for normal pages
> > and the destination pages can be in the same node with the source pages
> > (new page allocation from the same memblock are prohibited.)
> 
> No. It can't. memory hotplug change buddy attribute to MIGRATE_ISOLTE at first.
> then alloc_page() never allocate from source node. however huge page don't use
> buddy. then we need another trick.

MIGRATE_ISOLTE is changed only within the range [start_pfn, end_pfn)
given as the argument of __offline_pages (see also start_isolate_page_range),
so it's set only for pages within the single memblock to be offlined.

BTW, in previous discussion I already agreed with checking migrate type
in hugepage allocation code (maybe it will be in dequeue_huge_page_vma(),)
so what you concern should be solved in the next post.

> 
> > So if we want to avoid new page allocation from the same node,
> > this is the problem both for normal and huge pages.
> >
> > BTW, is it correct to think that all users of memory hotplug assume
> > that they want to hotplug a whole node (not the part of it?)
> 
> Both are valid use case. admin can isolate a part of memory for isolating
> broken memory range.
> 
> but I'm sure almost user want to remove whole node.

OK. So I think about "allocation in the nearest neighbor node",
although it can be in separate patch if it's hard to implement.

Thanks,
Naoya
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