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Recently a customer of ours experienced a crash when booting the
system while enabling memory-hotplug.

The problem is that Normal zones on different nodes don't get their private
zone->pageset allocated, and keep sharing the initial boot_pageset.
The sharing between zones is normally safe as explained by the comment for
boot_pageset - it's a percpu structure, and manipulations are done with
disabled interrupts, and boot_pageset is set up in a way that any page placed
on its pcplist is immediately flushed to shared zone's freelist, because
pcp->high == 1.
However, the hotplug operation updates pcp->high to a higher value as it
expects to be operating on a private pageset.

The problem is in build_all_zonelists(), which is called when the first range
of pages is onlined for the Normal zone of node X or Y:

	if (system_state == SYSTEM_BOOTING) {
		build_all_zonelists_init();
	} else {
	#ifdef CONFIG_MEMORY_HOTPLUG
		if (zone)
			setup_zone_pag...