[RFC][PATCH v3] slub: Keep page and object in sync in slab_alloc_node()

[Steven Rostedt <rostedt@goodmis.org>]

In slab_alloc_node(), after the cpu_slab is assigned, if the task is
preempted and moves to another CPU, there's nothing keeping the page and
object in sync. The -rt kernel crashed because page was NULL and object
was not, and the node_match() dereferences page. Even though the crash
happened on -rt, there's nothing that's keeping this from happening on
mainline.

The easiest fix is to disable preemption for the entire time from
acquiring the current CPU cpu_slab and assigning the object and page.
After that, it's fine to allow preemption.

Also add a check if page is NULL in node_match().

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/mm/slub.c b/mm/slub.c
index ba2ca53..10714ee 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2041,7 +2041,7 @@ static void flush_all(struct kmem_cache *s)
 static inline int node_match(struct page *page, int node)
 {
 #ifdef CONFIG_NUMA
-	if (node != NUMA_NO_NODE && page_to_nid(page) != node)
+	if (!page || (node != NUMA_NO_NODE && page_to_nid(page) != node))
 		return 0;
 #endif
 	return 1;
@@ -2337,7 +2337,10 @@ redo:
 	 * enabled. We may switch back and forth between cpus while
 	 * reading from one cpu area. That does not matter as long
 	 * as we end up on the original cpu again when doing the cmpxchg.
+	 *
+	 * But we need to sync the setting of page and object.
 	 */
+	preempt_disable();
 	c = __this_cpu_ptr(s->cpu_slab);
 
 	/*
@@ -2347,10 +2350,14 @@ redo:
 	 * linked list in between.
 	 */
 	tid = c->tid;
+
+	/* Must have tid first in case an interrupt comes in */
 	barrier();
 
 	object = c->freelist;
 	page = c->page;
+	preempt_enable();
+
 	if (unlikely(!object || !node_match(page, node)))
 		object = __slab_alloc(s, gfpflags, node, addr, c);
 


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