Re: [PATCH] mm/slub: fix a BUG_ON() when offlining a memory node and CONFIG_SLUB_DEBUG is on

[Christoph Lameter <cl@linux.com>]

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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