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authorVlastimil Babka <vbabka@suse.cz>2021-02-24 12:01:12 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2021-02-24 13:38:27 -0800
commit7e1fa93deff44677a94dfc323ff629bbf5cf9360 (patch)
tree693222436aafe91bdab6e78d1d0b916d139431fa
parent666716fd267df0007dfbb6480cd79dd5b05da4cc (diff)
mm, slab, slub: stop taking memory hotplug lock
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(). We however cannot simply restore slab_mutex in kmem_cache_shrink(), as SLUB can enters the function from a write to sysfs 'shrink' file, thus holding kernfs lock, and in kmem_cache_create() the kernfs lock is nested within slab_mutex. But on closer inspection we don't actually need to protect kmem_cache_shrink() from hotplug callbacks: While SLUB's __kmem_cache_shrink() does for_each_kmem_cache_node(), missing a new node added in parallel hotplug is not fatal, and parallel hotremove does not free kmem_cache_node's anymore after the previous patch, so use-after free cannot happen. The per-node shrinking itself is protected by n->list_lock. Same is true for SLAB, and SLOB is no-op. 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. Link: https://lkml.kernel.org/r/20210113131634.3671-3-vbabka@suse.cz Signed-off-by: Vlastimil Babka <vbabka@suse.cz> Cc: Christoph Lameter <cl@linux.com> Cc: David Hildenbrand <david@redhat.com> Cc: David Rientjes <rientjes@google.com> Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Pekka Enberg <penberg@kernel.org> Cc: Qian Cai <cai@redhat.com> Cc: Vladimir Davydov <vdavydov.dev@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/slab_common.c8
-rw-r--r--mm/slub.c28
2 files changed, 27 insertions, 9 deletions
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 580124b7fed8..d73bea53f759 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -310,7 +310,6 @@ kmem_cache_create_usercopy(const char *name,
int err;
get_online_cpus();
- get_online_mems();
mutex_lock(&slab_mutex);
@@ -360,7 +359,6 @@ kmem_cache_create_usercopy(const char *name,
out_unlock:
mutex_unlock(&slab_mutex);
- put_online_mems();
put_online_cpus();
if (err) {
@@ -487,7 +485,6 @@ void kmem_cache_destroy(struct kmem_cache *s)
return;
get_online_cpus();
- get_online_mems();
mutex_lock(&slab_mutex);
@@ -504,7 +501,6 @@ 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);
@@ -523,10 +519,10 @@ int kmem_cache_shrink(struct kmem_cache *cachep)
int ret;
get_online_cpus();
- get_online_mems();
+
kasan_cache_shrink(cachep);
ret = __kmem_cache_shrink(cachep);
- put_online_mems();
+
put_online_cpus();
return ret;
}
diff --git a/mm/slub.c b/mm/slub.c
index 1ecd8a7bd25c..175cd905b58a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -235,6 +235,14 @@ 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 @@ redo:
* 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 {
@@ -3592,7 +3600,7 @@ 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) {
@@ -4286,6 +4294,7 @@ static void slab_mem_offline_callback(void *arg)
return;
mutex_lock(&slab_mutex);
+ node_clear(offline_node, slab_nodes);
/*
* We no longer free kmem_cache_node structures here, as it would be
* racy with all get_node() users, and infeasible to protect them with
@@ -4335,6 +4344,11 @@ 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;
@@ -4415,6 +4429,7 @@ 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;
@@ -4422,6 +4437,13 @@ 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);