linux-kernel - Re: [PATCH] mm/slub: fix a BUG_ON() when offlining a memory node and CONFIG_SLUB

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Message-ID: <alpine.DEB.2.00.1207181349370.22907@router.home>
Date:	Wed, 18 Jul 2012 13:53:15 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jiang Liu <liuj97@...il.com>
cc:	Pekka Enberg <penberg@...nel.org>, Matt Mackall <mpm@...enic.com>,
	Mel Gorman <mgorman@...e.de>,
	Jianguo Wu <wujianguo@...wei.com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Tony Luck <tony.luck@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Keping Chen <chenkeping@...wei.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slub: fix a BUG_ON() when offlining a memory node
 and CONFIG_SLUB_DEBUG is on

On Thu, 19 Jul 2012, Jiang Liu wrote:

> 	I found the previous analysis of the BUG_ON() issue is incorrect after
> another round of code review.
> 	The really issue is that function early_kmem_cache_node_alloc() calls
> inc_slabs_node(kmem_cache_node, node, page->objects) to increase the object
> count on local node no matter whether page is allocated from local or remote
> node. With current implementation it's OK because every memory node has normal
> memory so page is allocated from local node. Now we are working on a patch set
> to improve memory hotplug. The basic idea is to to let some memory nodes only
> host ZONE_MOVABLE zone, so we could easily remove the whole memory node when
> needed. That means some memory nodes have no ZONE_NORMAL/ZONE_DMA, and the page
> will be allocated from remote node in function early_kmem_cache_node_alloc().
> But early_kmem_cache_node_alloc() still increases object count on local node,
> which triggers the BUG_ON eventually when removing the affected memory node.

That does not work. If the node does only have ZONE_MOVABLE then no slab
object can be allocated from the zone. You need to modify the slab
allocators to not allocate a per node structure for those zones and forbit
all allocations from such a node. Actually that should already work
because only ZONE_NORMAL nodes should get a per node structure because
slab objects can only be allocated from ZONE_NORMAL.


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