mm/slab.c: fix SLAB freelist randomization duplicate entries

This patch fixes a bug in the freelist randomization code.  When a high
random number is used, the freelist will contain duplicate entries.  It
will result in different allocations sharing the same chunk.

It will result in odd behaviours and crashes.  It should be uncommon but
it depends on the machines.  We saw it happening more often on some
machines (every few hours of running tests).

Fixes: c7ce4f60ac19 ("mm: SLAB freelist randomization")
Link: http://lkml.kernel.org/r/20170103181908.143178-1-thgarnie@google.com
Signed-off-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Thomas Garnier <thgarnie@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
void ___cache_free(struct kmem_cache *cachep, void *objp,
		unsigned long caller)
{
	struct array_cache *ac = cpu_cache_get(cachep);

	check_irq_off();
	kmemleak_free_recursive(objp, cachep->flags);
	objp = cache_free_debugcheck(cachep, objp, caller);

	kmemcheck_slab_free(cachep, objp, cachep->object_size);

	/*
	 * Skip calling cache_free_alien() when the platform is not numa.
	 * This will avoid cache misses that happen while accessing slabp (which
	 * is per page memory  reference) to get nodeid. Instead use a global
	 * variable to skip the call, which is mostly likely to be present in
	 * the cache.
	 */
	if (nr_online_nodes > 1 && cache_free_alien(cachep, objp))
		return;

	if (ac->avail < ac->limit) {
		STATS_INC_FREEHIT(cachep);
	} else {
		STATS_INC_FREEMISS(cachep);
		cache_flusharray(cachep, ac);
	}

	if (sk_memalloc_socks()) {
		struct page *page = virt_to_head_page(objp);

		if (unlikely(PageSlabPfmemalloc(page))) {
			cache_free_pfmemalloc(cachep, page, objp);
			return;
		}
	}

	ac->entry[ac->avail++] = objp;
}

void ___cache_free(struct kmem_cache *cachep, void *objp,
		unsigned long caller)
{
	struct array_cache *ac = cpu_cache_get(cachep);

	check_irq_off();
	kmemleak_free_recursive(objp, cachep->flags);
	objp = cache_free_debugcheck(cachep, objp, caller);

	kmemcheck_slab_free(cachep, objp, cachep->object_size);

	/*
	 * Skip calling cache_free_alien() when the platform is not numa.
	 * This will avoid cache misses that happen while accessing slabp (which
	 * is per page memory  reference) to get nodeid. Instead use a global
	 * variable to skip the call, which is mostly likely to be present in
	 * the cache.
	 */
	if (nr_online_nodes > 1 && cache_free_alien(cachep, objp))
		return;

	if (ac->avail < ac->limit) {
		STATS_INC_FREEHIT(cachep);
	} else {
		STATS_INC_FREEMISS(cachep);
		cache_flusharray(cachep, ac);
	}

	if (sk_memalloc_socks()) {
		struct page *page = virt_to_head_page(objp);

		if (unlikely(PageSlabPfmemalloc(page))) {
			cache_free_pfmemalloc(cachep, page, objp);
			return;
		}
	}

	ac->entry[ac->avail++] = objp;
}
