[PATCH] mm/slab.c: fix early init assumption

The SLAB bootstrap code assumes that the first two kmalloc caches created
(the INDEX_AC and INDEX_L3 kmalloc caches) wont be off-slab.  But due to AC
and L3 structure size increase in lockdep, one of them ended up being
off-slab, and subsequently crashing with:

Unable to handle kernel NULL pointer dereference at 0000000000000000 RIP:
 [<ffffffff80267478>] kmem_cache_alloc+0x26/0x7d

The fix is to introduce a bootstrap flag and to use it to prevent off-slab
caches being created so early during bootup.

(The calculation for off-slab caches is quite complex so i didnt want to
complicate things with introducing yet another INDEX_ calculation, the flag
approach is simpler and smaller.)
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/slab.c

@@ -331,6 +331,8 @@ static __always_inline int index_of(const size_t size)
 	return 0;
 }
 
+static int slab_early_init = 1;
+
 #define INDEX_AC index_of(sizeof(struct arraycache_init))
 #define INDEX_L3 index_of(sizeof(struct kmem_list3))
 
@@ -1376,6 +1378,8 @@ void __init kmem_cache_init(void)
 				NULL, NULL);
 	}
 
+	slab_early_init = 0;
+
 	while (sizes->cs_size != ULONG_MAX) {
 		/*
 		 * For performance, all the general caches are L1 aligned.
@@ -2106,8 +2110,12 @@ kmem_cache_create (const char *name, size_t size, size_t align,
 #endif
 #endif
 
-	/* Determine if the slab management is 'on' or 'off' slab. */
-	if (size >= (PAGE_SIZE >> 3))
+	/*
+	 * Determine if the slab management is 'on' or 'off' slab.
+	 * (bootstrapping cannot cope with offslab caches so don't do
+	 * it too early on.)
+	 */
+	if ((size >= (PAGE_SIZE >> 3)) && !slab_early_init)
 		/*
 		 * Size is large, assume best to place the slab management obj
 		 * off-slab (should allow better packing of objs).