NUMA slab allocator migration bugfix

NUMA slab allocator cpu migration bugfix

The NUMA slab allocator (specifically, cache_alloc_refill) is not refreshing its local copies of what cpu and what numa node it is on, when it drops and reacquires the irq block that it inherited from its caller. As a result those values become invalid if an attempt to migrate the process to another numa node occured while the irq block had been dropped.

The solution is to make cache_alloc_refill reload these variables whenever it drops and reacquires the irq block.

The error is very difficult to hit. When it does occur, one gets the following oops + stack traceback bits in check_spinlock_acquired:

	kernel BUG at mm/slab.c:2417
	cache_alloc_refill+0xe6
	kmem_cache_alloc+0xd0
	...

This patch was developed against 2.6.23, ported to and compiled-tested only against 2.6.25-rc4.

Index: 2.6.25-rc4/mm/slab.c
===================================================================
@@ -2964,11 +2964,10 @@
 	struct array_cache *ac;
 	int node;
 
-	node = numa_node_id();
-
+retry:
 	check_irq_off();
+	node = numa_node_id();
 	ac = cpu_cache_get(cachep);
-retry:
 	batchcount = ac->batchcount;
 	if (!ac->touched && batchcount > BATCHREFILL_LIMIT) {
 		/*

The new slab is allocated for the node that was determined earlier and entered into the slab queues for that node. Howver, during the alloc we were rescheduled.

Then we find ourselves on another processor and recalculate the ac pointer. If we now retry then there is the danger of getting off node objects into the per cpu queue. Which may cause the wrong lock to be taken when draining queues. Sucks because it can cause data corruption. Same as the other issues resolved by GFP_THISNODE.

Will queue it.