slab: get_online_mems for kmem_cache_{create,destroy,shrink}

When we create a sl[au]b cache, we allocate kmem_cache_node structures
for each online NUMA node.  To handle nodes taken online/offline, we
register memory hotplug notifier and allocate/free kmem_cache_node
corresponding to the node that changes its state for each kmem cache.

To synchronize between the two paths we hold the slab_mutex during both
the cache creationg/destruction path and while tuning per-node parts of
kmem caches in memory hotplug handler, but that's not quite right,
because it does not guarantee that a newly created cache will have all
kmem_cache_nodes initialized in case it races with memory hotplug.  For
instance, in case of slub:

    CPU0                            CPU1
    ----                            ----
    kmem_cache_create:              online_pages:
     __kmem_cache_create:            slab_memory_callback:
                                      slab_mem_going_online_callback:
                                       lock slab_mutex
                                       for each slab_caches list entry
                                           allocate kmem_cache node
                                       unlock slab_mutex
      lock slab_mutex
      init_kmem_cache_nodes:
       for_each_node_state(node, N_NORMAL_MEMORY)
           allocate kmem_cache node
      add kmem_cache to slab_caches list
      unlock slab_mutex
                                    online_pages (continued):
                                     node_states_set_node

As a result we'll get a kmem cache with not all kmem_cache_nodes
allocated.

To avoid issues like that we should hold get/put_online_mems() during
the whole kmem cache creation/destruction/shrink paths, just like we
deal with cpu hotplug.  This patch does the trick.

Note, that after it's applied, there is no need in taking the slab_mutex
for kmem_cache_shrink any more, so it is removed from there.

Signed-off-by: Vladimir Davydov <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: Tang Chen <[email protected]>
Cc: Zhang Yanfei <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: Xishi Qiu <[email protected]>
Cc: Jiang Liu <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Wen Congyang <[email protected]>
Cc: Yasuaki Ishimatsu <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/slob.c

@@ -620,11 +620,10 @@ int __kmem_cache_shutdown(struct kmem_cache *c)
 	return 0;
 }
 
-int kmem_cache_shrink(struct kmem_cache *d)
+int __kmem_cache_shrink(struct kmem_cache *d)
 {
 	return 0;
 }
-EXPORT_SYMBOL(kmem_cache_shrink);
 
 struct kmem_cache kmem_cache_boot = {
 	.name = "kmem_cache",