On Thu, 24 Nov 2011, Mel Gorman wrote:

quoted

Calling alloc_pages_exact_node() means the allocation only passes the
zonelist of a single node into the page allocator.  If that node isn't
online, it's zonelist may never have been initialized causing a strange
oops that may not immediately be clear.

I recently debugged an issue where node 0 wasn't online and an allocator
was passing 0 to alloc_pages_exact_node() and it resulted in a NULL
pointer on zonelist->_zoneref.  If CONFIG_DEBUG_VM is enabled, though, it
would be nice to catch this a bit earlier.

Signed-off-by: David Rientjes <rientjes@google.com>

Acked-by: Mel Gorman <mgorman@suse.de>

Out of curiousity, who was passing in the ID of an offline node to
alloc_pages_exact_node() ?

It was the block layer in blk_throtl_init() because it passes the ->node 
field of request_queue to the slab layer which uses 
alloc_pages_exact_node() and requeue_queue is allocated with __GFP_ZERO 
and ->node isn't initialized until later.  At the same time, the machine 
only has a single node online, node 1, where the crashkernel was 
allocated.  My analysis is at 
http://marc.info/?l=linux-kernel&m=132195611123426

I've worked with kernels without a node 0 very successfully since about 
2.6.18 so the VM appears pretty stable in that regard, too, which is good 
news.

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