[RFC 1/3] mm, slab, slub: stop taking memory hotplug lock

[Vlastimil Babka]

Since commit 03afc0e25f7f ("slab: get_online_mems for
kmem_cache_{create,destroy,shrink}") we are taking memory hotplug lock for SLAB
and SLUB when creating, destroying or shrinking a cache. It is quite a heavy
lock and it's best to avoid it if possible, as we had several issues with
lockdep complaining about ordering in the past, see e.g. e4f8e513c3d3
("mm/slub: fix a deadlock in show_slab_objects()").

The problem scenario in 03afc0e25f7f (solved by the memory hotplug lock) can be
summarized as follows: while there's slab_mutex synchronizing new kmem cache
creation and SLUB's MEM_GOING_ONLINE callback slab_mem_going_online_callback(),
we may miss creation of kmem_cache_node for the hotplugged node in the new kmem
cache, because the hotplug callback doesn't yet see the new cache, and cache
creation in init_kmem_cache_nodes() only inits kmem_cache_node for nodes in the
N_NORMAL_MEMORY nodemask, which however may not yet include the new node, as
that happens only later after the MEM_GOING_ONLINE callback.

Instead of using get/put_online_mems(), the problem can be solved by SLUB
maintaining its own nodemask of nodes for which it has allocated the per-node
kmem_cache_node structures. This nodemask would generally mirror the
N_NORMAL_MEMORY nodemask, but would be updated only in under SLUB's control in
its memory hotplug callbacks under the slab_mutex. This patch adds such
nodemask and its handling.

Commit 03afc0e25f7f mentiones "issues like [the one above]", but there don't
appear to be further issues. All the paths (shared for SLAB and SLUB) taking
the memory hotplug locks are also taking the slab_mutex, except
kmem_cache_shrink() where 03afc0e25f7f replaced slab_mutex with
get/put_online_mems(). So this patch restores slab_mutex in
kmem_cache_shrink(). slab_mutex should be otherwise sufficient, as all the
memory hotplug callbacks in SLUB take it as well.

SLAB also doesn't need the memory hotplug locking, which it only gained by
03afc0e25f7f through the shared paths in slab_common.c. Its memory hotplug
callbacks are also protected by slab_mutex against races with these paths. The
problem of SLUB relying on N_NORMAL_MEMORY doesn't apply to SLAB, as its
setup_kmem_cache_nodes relies on N_ONLINE, and the new node is already set
there during the MEM_GOING_ONLINE callback, so no special care is needed
for SLAB.

As such, this patch removes all get/put_online_mems() usage by the slab
subsystem.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/slab_common.c | 10 ++++------
 mm/slub.c        | 28 +++++++++++++++++++++++++---
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 075b23ce94ec..e728265c8b7d 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -311,7 +311,6 @@ kmem_cache_create_usercopy(const char *name,
 	int err;
 
 	get_online_cpus();
-	get_online_mems();
 
 	mutex_lock(&slab_mutex);
 
@@ -361,7 +360,6 @@ kmem_cache_create_usercopy(const char *name,
 out_unlock:
 	mutex_unlock(&slab_mutex);
 
-	put_online_mems();
 	put_online_cpus();
 
 	if (err) {
@@ -490,7 +488,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
 		return;
 
 	get_online_cpus();
-	get_online_mems();
 
 	mutex_lock(&slab_mutex);
 
@@ -507,7 +504,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
 out_unlock:
 	mutex_unlock(&slab_mutex);
 
-	put_online_mems();
 	put_online_cpus();
 }
 EXPORT_SYMBOL(kmem_cache_destroy);
@@ -526,10 +522,12 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
 	int ret;
 
 	get_online_cpus();
-	get_online_mems();
+	mutex_lock(&slab_mutex);
+
 	kasan_cache_shrink(cachep);
 	ret = __kmem_cache_shrink(cachep);
-	put_online_mems();
+
+	mutex_unlock(&slab_mutex);
 	put_online_cpus();
 	return ret;
 }
diff --git a/mm/slub.c b/mm/slub.c
index 1f4584954f4c..2e2edd5c9cfc 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -236,6 +236,14 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
 #endif
 }
 
+/*
+ * Tracks for which NUMA nodes we have kmem_cache_nodes allocated.
+ * Corresponds to node_state[N_NORMAL_MEMORY], but can temporarily
+ * differ during memory hotplug/hotremove operations.
+ * Protected by slab_mutex.
+ */
+static nodemask_t slab_nodes;
+
 /********************************************************************
  * 			Core slab cache functions
  *******************************************************************/
@@ -2678,7 +2686,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		 * ignore the node constraint
 		 */
 		if (unlikely(node != NUMA_NO_NODE &&
-			     !node_state(node, N_NORMAL_MEMORY)))
+			     !node_isset(node, slab_nodes)))
 			node = NUMA_NO_NODE;
 		goto new_slab;
 	}
@@ -2689,7 +2697,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		 * same as above but node_match() being false already
 		 * implies node != NUMA_NO_NODE
 		 */
-		if (!node_state(node, N_NORMAL_MEMORY)) {
+		if (!node_isset(node, slab_nodes)) {
 			node = NUMA_NO_NODE;
 			goto redo;
 		} else {
@@ -3599,7 +3607,7 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
 {
 	int node;
 
-	for_each_node_state(node, N_NORMAL_MEMORY) {
+	for_each_node_mask(node, slab_nodes) {
 		struct kmem_cache_node *n;
 
 		if (slab_state == DOWN) {
@@ -4259,6 +4267,7 @@ static void slab_mem_offline_callback(void *arg)
 		return;
 
 	mutex_lock(&slab_mutex);
+	node_clear(offline_node, slab_nodes);
 	list_for_each_entry(s, &slab_caches, list) {
 		n = get_node(s, offline_node);
 		if (n) {
@@ -4312,6 +4321,11 @@ static int slab_mem_going_online_callback(void *arg)
 		init_kmem_cache_node(n);
 		s->node[nid] = n;
 	}
+	/*
+	 * Any cache created after this point will also have kmem_cache_node
+	 * initialized for the new node.
+	 */
+	node_set(nid, slab_nodes);
 out:
 	mutex_unlock(&slab_mutex);
 	return ret;
@@ -4392,6 +4406,7 @@ void __init kmem_cache_init(void)
 {
 	static __initdata struct kmem_cache boot_kmem_cache,
 		boot_kmem_cache_node;
+	int node;
 
 	if (debug_guardpage_minorder())
 		slub_max_order = 0;
@@ -4399,6 +4414,13 @@ void __init kmem_cache_init(void)
 	kmem_cache_node = &boot_kmem_cache_node;
 	kmem_cache = &boot_kmem_cache;
 
+	/*
+	 * Initialize the nodemask for which we will allocate per node
+	 * structures. Here we don't need taking slab_mutex yet.
+	 */
+	for_each_node_state(node, N_NORMAL_MEMORY)
+		node_set(node, slab_nodes);
+
 	create_boot_cache(kmem_cache_node, "kmem_cache_node",
 		sizeof(struct kmem_cache_node), SLAB_HWCACHE_ALIGN, 0, 0);
 
-- 
2.29.2