mm, mempool: disallow mempools based on slab caches with constructors

All occurrences of mempools based on slab caches with object constructors
have been removed from the tree, so disallow creating them.

We can only dereference mem->ctor in mm/mempool.c without including
mm/slab.h in include/linux/mempool.h.  So simply note the restriction,
just like the comment restricting usage of __GFP_ZERO, and warn on kernels
with CONFIG_DEBUG_VM() if such a mempool is allocated from.

We don't want to incur this check on every element allocation, so use
VM_BUG_ON().

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Dave Kleikamp <shaggy@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sebastian Ott <sebott@linux.vnet.ibm.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: rientjes

include/linux/mempool.h

@@ -36,7 +36,8 @@ extern void mempool_free(void *element, mempool_t *pool);
 
 /*
  * A mempool_alloc_t and mempool_free_t that get the memory from
- * a slab that is passed in through pool_data.
+ * a slab cache that is passed in through pool_data.
+ * Note: the slab cache may not have a ctor function.
  */
 void *mempool_alloc_slab(gfp_t gfp_mask, void *pool_data);
 void mempool_free_slab(void *element, void *pool_data);

mm/mempool.c

@@ -15,6 +15,7 @@
 #include <linux/mempool.h>
 #include <linux/blkdev.h>
 #include <linux/writeback.h>
+#include "slab.h"
 
 static void add_element(mempool_t *pool, void *element)
 {
@@ -334,6 +335,7 @@ EXPORT_SYMBOL(mempool_free);
 void *mempool_alloc_slab(gfp_t gfp_mask, void *pool_data)
 {
 	struct kmem_cache *mem = pool_data;
+	VM_BUG_ON(mem->ctor);
 	return kmem_cache_alloc(mem, gfp_mask);
 }
 EXPORT_SYMBOL(mempool_alloc_slab);