slab: __GFP_ZERO is incompatible with a constructor

__GFP_ZERO requests that the object be initialised to all-zeroes, while
the purpose of a constructor is to initialise an object to a particular
pattern.  We cannot do both.  Add a warning to catch any users who
mistakenly pass a __GFP_ZERO flag when allocating a slab with a
constructor.

Link: http://lkml.kernel.org/r/20180412191322.GA21205@bombadil.infradead.org
Fixes: d07dbea464 ("Slab allocators: support __GFP_ZERO in all allocators")
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -2444,6 +2444,8 @@ static inline void *new_slab_objects(struct kmem_cache *s, gfp_t flags,
 	struct kmem_cache_cpu *c = *pc;
 	struct page *page;
 
+	WARN_ON_ONCE(s->ctor && (flags & __GFP_ZERO));
+
 	freelist = get_partial(s, flags, node, c);
 
 	if (freelist)