[PATCH] mm: fix race between kmem_cache destroy, create and deactivate

From: Shakeel Butt
Date: Mon May 21 2018 - 12:47:43 EST

The memcg kmem cache creation and deactivation (SLUB only) is
asynchronous. If a root kmem cache is destroyed whose memcg cache is in
the process of creation or deactivation, the kernel may crash.

Example of one such crash:
general protection fault: 0000 [#1] SMP PTI
CPU: 1 PID: 1721 Comm: kworker/14:1 Not tainted 4.17.0-smp
...
Workqueue: memcg_kmem_cache kmemcg_deactivate_workfn
RIP: 0010:has_cpu_slab
...
Call Trace:
? on_each_cpu_cond
__kmem_cache_shrink
kmemcg_cache_deact_after_rcu
kmemcg_deactivate_workfn
process_one_work
worker_thread
kthread
ret_from_fork+0x35/0x40

This issue is due to the lack of reference counting for the root
kmem_caches. There exist a refcount in kmem_cache but it is actually a
count of aliases i.e. number of kmem_caches merged together.

This patch make alias count explicit and adds reference counting to the
root kmem_caches. The reference of a root kmem cache is elevated on
merge and while its memcg kmem_cache is in the process of creation or
deactivation.

Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
include/linux/slab.h | 2 +
include/linux/slab_def.h | 3 +-
include/linux/slub_def.h | 3 +-
mm/memcontrol.c | 7 ++++
mm/slab.c | 4 +-
mm/slab.h | 5 ++-
mm/slab_common.c | 84 ++++++++++++++++++++++++++++++----------
mm/slub.c | 14 ++++---
8 files changed, 90 insertions(+), 32 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 9ebe659bd4a5..4c28f2483a22 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -674,6 +674,8 @@ struct memcg_cache_params {
};

int memcg_update_all_caches(int num_memcgs);
+bool kmem_cache_tryget(struct kmem_cache *s);
+void kmem_cache_put(struct kmem_cache *s);

/**
* kmalloc_array - allocate memory for an array.
diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
index d9228e4d0320..4bb22c89a740 100644
--- a/include/linux/slab_def.h
+++ b/include/linux/slab_def.h
@@ -41,7 +41,8 @@ struct kmem_cache {
/* 4) cache creation/removal */
const char *name;
struct list_head list;
- int refcount;
+ refcount_t refcount;
+ int alias_count;
int object_size;
int align;

diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index 3773e26c08c1..532d4b6f83ed 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -97,7 +97,8 @@ struct kmem_cache {
struct kmem_cache_order_objects max;
struct kmem_cache_order_objects min;
gfp_t allocflags; /* gfp flags to use on each alloc */
- int refcount; /* Refcount for slab cache destroy */
+ refcount_t refcount; /* Refcount for slab cache destroy */
+ int alias_count; /* Number of root kmem caches merged */
void (*ctor)(void *);
unsigned int inuse; /* Offset to metadata */
unsigned int align; /* Alignment */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index bdb8028c806c..ab5673dbfc4e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2185,6 +2185,7 @@ static void memcg_kmem_cache_create_func(struct work_struct *w)
memcg_create_kmem_cache(memcg, cachep);

css_put(&memcg->css);
+ kmem_cache_put(cachep);
kfree(cw);
}

@@ -2200,6 +2201,12 @@ static void __memcg_schedule_kmem_cache_create(struct mem_cgroup *memcg,
if (!cw)
return;

+ /* Make sure root kmem cache does not get destroyed in the middle */
+ if (!kmem_cache_tryget(cachep)) {
+ kfree(cw);
+ return;
+ }
+
css_get(&memcg->css);

cw->memcg = memcg;
diff --git a/mm/slab.c b/mm/slab.c
index c1fe8099b3cd..080732f5f20d 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1883,8 +1883,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
struct kmem_cache *cachep;

cachep = find_mergeable(size, align, flags, name, ctor);
- if (cachep) {
- cachep->refcount++;
+ if (cachep && kmem_cache_tryget(cachep)) {
+ cachep->alias_count++;

/*
* Adjust the object sizes so that we clear
diff --git a/mm/slab.h b/mm/slab.h
index 68bdf498da3b..25962ab75ec1 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -25,7 +25,8 @@ struct kmem_cache {
unsigned int useroffset;/* Usercopy region offset */
unsigned int usersize; /* Usercopy region size */
const char *name; /* Slab name for sysfs */
- int refcount; /* Use counter */
+ refcount_t refcount; /* Use counter */
+ int alias_count;
void (*ctor)(void *); /* Called on object slot creation */
struct list_head list; /* List of all slab caches on the system */
};
@@ -295,7 +296,7 @@ extern void slab_init_memcg_params(struct kmem_cache *);
extern void memcg_link_cache(struct kmem_cache *s);
extern void slab_deactivate_memcg_cache_rcu_sched(struct kmem_cache *s,
void (*deact_fn)(struct kmem_cache *));
-
+extern void kmem_cache_put_locked(struct kmem_cache *s);
#else /* CONFIG_MEMCG && !CONFIG_SLOB */

/* If !memcg, all caches are root. */
diff --git a/mm/slab_common.c b/mm/slab_common.c
index b0dd9db1eb2f..390eb47486fd 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -306,7 +306,7 @@ int slab_unmergeable(struct kmem_cache *s)
/*
* We may have set a slab to be unmergeable during bootstrap.
*/
- if (s->refcount < 0)
+ if (s->alias_count < 0)
return 1;

return 0;
@@ -391,7 +391,8 @@ static struct kmem_cache *create_cache(const char *name,
if (err)
goto out_free_cache;

- s->refcount = 1;
+ s->alias_count = 1;
+ refcount_set(&s->refcount, 1);
list_add(&s->list, &slab_caches);
memcg_link_cache(s);
out:
@@ -611,6 +612,13 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
if (memcg->kmem_state != KMEM_ONLINE)
goto out_unlock;

+ /*
+ * The root cache has been requested to be destroyed while its memcg
+ * cache was in creation queue.
+ */
+ if (!root_cache->alias_count)
+ goto out_unlock;
+
idx = memcg_cache_id(memcg);
arr = rcu_dereference_protected(root_cache->memcg_params.memcg_caches,
lockdep_is_held(&slab_mutex));
@@ -663,6 +671,8 @@ static void kmemcg_deactivate_workfn(struct work_struct *work)
{
struct kmem_cache *s = container_of(work, struct kmem_cache,
memcg_params.deact_work);
+ struct kmem_cache *root = s->memcg_params.root_cache;
+ struct mem_cgroup *memcg = s->memcg_params.memcg;

get_online_cpus();
get_online_mems();
@@ -677,7 +687,8 @@ static void kmemcg_deactivate_workfn(struct work_struct *work)
put_online_cpus();

/* done, put the ref from slab_deactivate_memcg_cache_rcu_sched() */
- css_put(&s->memcg_params.memcg->css);
+ css_put(&memcg->css);
+ kmem_cache_put(root);
}

static void kmemcg_deactivate_rcufn(struct rcu_head *head)
@@ -712,6 +723,10 @@ void slab_deactivate_memcg_cache_rcu_sched(struct kmem_cache *s,
WARN_ON_ONCE(s->memcg_params.deact_fn))
return;

+ /* Make sure root kmem_cache does not get destroyed in the middle */
+ if (!kmem_cache_tryget(s->memcg_params.root_cache))
+ return;
+
/* pin memcg so that @s doesn't get destroyed in the middle */
css_get(&s->memcg_params.memcg->css);

@@ -838,21 +853,17 @@ void slab_kmem_cache_release(struct kmem_cache *s)
kmem_cache_free(kmem_cache, s);
}

-void kmem_cache_destroy(struct kmem_cache *s)
+static void __kmem_cache_destroy(struct kmem_cache *s, bool lock)
{
int err;

- if (unlikely(!s))
- return;
-
- get_online_cpus();
- get_online_mems();
+ if (lock) {
+ get_online_cpus();
+ get_online_mems();
+ mutex_lock(&slab_mutex);
+ }

- mutex_lock(&slab_mutex);
-
- s->refcount--;
- if (s->refcount)
- goto out_unlock;
+ VM_BUG_ON(s->alias_count);

err = shutdown_memcg_caches(s);
if (!err)
@@ -863,11 +874,42 @@ void kmem_cache_destroy(struct kmem_cache *s)
s->name);
dump_stack();
}
-out_unlock:
- mutex_unlock(&slab_mutex);

- put_online_mems();
- put_online_cpus();
+ if (lock) {
+ mutex_unlock(&slab_mutex);
+ put_online_mems();
+ put_online_cpus();
+ }
+}
+
+bool kmem_cache_tryget(struct kmem_cache *s)
+{
+ if (is_root_cache(s))
+ return refcount_inc_not_zero(&s->refcount);
+ return false;
+}
+
+void kmem_cache_put(struct kmem_cache *s)
+{
+ if (is_root_cache(s) &&
+ refcount_dec_and_test(&s->refcount))
+ __kmem_cache_destroy(s, true);
+}
+
+void kmem_cache_put_locked(struct kmem_cache *s)
+{
+ if (is_root_cache(s) &&
+ refcount_dec_and_test(&s->refcount))
+ __kmem_cache_destroy(s, false);
+}
+
+void kmem_cache_destroy(struct kmem_cache *s)
+{
+ if (unlikely(!s))
+ return;
+
+ s->alias_count--;
+ kmem_cache_put(s);
}
EXPORT_SYMBOL(kmem_cache_destroy);

@@ -919,7 +961,8 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
panic("Creation of kmalloc slab %s size=%u failed. Reason %d\n",
name, size, err);

- s->refcount = -1; /* Exempt from merging for now */
+ s->alias_count = -1; /* Exempt from merging for now */
+ refcount_set(&s->refcount, 1);
}

struct kmem_cache *__init create_kmalloc_cache(const char *name,
@@ -934,7 +977,8 @@ struct kmem_cache *__init create_kmalloc_cache(const char *name,
create_boot_cache(s, name, size, flags, useroffset, usersize);
list_add(&s->list, &slab_caches);
memcg_link_cache(s);
- s->refcount = 1;
+ s->alias_count = 1;
+ refcount_set(&s->refcount, 1);
return s;
}

diff --git a/mm/slub.c b/mm/slub.c
index 48f75872c356..2e45f7febc6e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4270,8 +4270,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
struct kmem_cache *s, *c;

s = find_mergeable(size, align, flags, name, ctor);
- if (s) {
- s->refcount++;
+ if (s && kmem_cache_tryget(s)) {
+ s->alias_count++;

/*
* Adjust the object sizes so that we clear
@@ -4286,7 +4286,8 @@ __kmem_cache_alias(const char *name, unsigned int size, unsigned int align,
}

if (sysfs_slab_alias(s, name)) {
- s->refcount--;
+ s->alias_count--;
+ kmem_cache_put_locked(s);
s = NULL;
}
}
@@ -5009,7 +5010,8 @@ SLAB_ATTR_RO(ctor);

static ssize_t aliases_show(struct kmem_cache *s, char *buf)
{
- return sprintf(buf, "%d\n", s->refcount < 0 ? 0 : s->refcount - 1);
+ return sprintf(buf, "%d\n",
+ s->alias_count < 0 ? 0 : s->alias_count - 1);
}
SLAB_ATTR_RO(aliases);

@@ -5162,7 +5164,7 @@ static ssize_t trace_store(struct kmem_cache *s, const char *buf,
* as well as cause other issues like converting a mergeable
* cache into an umergeable one.
*/
- if (s->refcount > 1)
+ if (s->alias_count > 1)
return -EINVAL;

s->flags &= ~SLAB_TRACE;
@@ -5280,7 +5282,7 @@ static ssize_t failslab_show(struct kmem_cache *s, char *buf)
static ssize_t failslab_store(struct kmem_cache *s, const char *buf,
size_t length)
{
- if (s->refcount > 1)
+ if (s->alias_count > 1)
return -EINVAL;

s->flags &= ~SLAB_FAILSLAB;
--
2.17.0.441.gb46fe60e1d-goog