[PATCH net-next 3/9] bpf: pre-allocate hash map elements

Alexei Starovoitov
2016-03-07T02:00:32Z

If kprobe is placed on spin_unlock then calling kmalloc/kfree from
bpf programs is not safe, since the following dead lock is possible:
kfree->spin_lock(kmem_cache_node->lock)...spin_unlock->kprobe->
bpf_prog->map_update->kmalloc->spin_lock(of the same kmem_cache_node->lock)
and deadlocks.

The following solutions were considered and some implemented, but
eventually discarded
- kmem_cache_create for every map
- add recursion check to slow-path of slub
- use reserved memory in bpf_map_update for in_irq or in preempt_disabled
- kmalloc via irq_work

At the end pre-allocation of all map elements turned out to be the simplest
solution and since the user is charged upfront for all the memory, such
pre-allocation doesn't affect the user space visible behavior.

Since it's impossible to tell whether kprobe is triggered in a safe
location from kmalloc point of view, use pre-allocation by default
and introduce new BPF_F_NO_PREALLOC flag.

While testing of per-cpu hash maps it was discovered
that alloc_percpu(GFP_ATOMIC) has odd corner cases and often
fails to allocate memory even when 90% of it is free.
The pre-allocation of per-cpu hash elements solves this problem as well.

Turned out that bpf_map_update() quickly followed by
bpf_map_lookup()+bpf_map_delete() is very common pattern used
in many of iovisor/bcc/tools, so there is additional benefit of
pre-allocation, since such use cases are must faster.

Since all hash map elements are now pre-allocated we can remove
atomic increment of htab->count and save few more cycles.

Also add precharge_memlock() to check rlimit_memlock early to avoid
large malloc/free done by by users who don't have sufficient limits.

Pre-allocation is done with vmalloc and alloc/free is done
via percpu_freelist. Here are performance numbers for different
pre-allocation algorithms that were implemented, but discarded
in favor of percpu_freelist:

1 cpu:
pcpu_ida 2.1M
pcpu_ida nolock 2.3M
bt 2.4M
kmalloc 1.8M
hlist+spinlock 2.3M
pcpu_freelist 2.6M

4 cpu:
pcpu_ida 1.5M
pcpu_ida nolock 1.8M
bt w/smp_align 1.7M
bt no/smp_align 1.1M
kmalloc 0.7M
hlist+spinlock 0.2M
pcpu_freelist 2.0M

8 cpu:
pcpu_ida 0.7M
bt w/smp_align 0.8M
kmalloc 0.4M
pcpu_freelist 1.5M

32 cpu:
kmalloc 0.13M
pcpu_freelist 0.49M

pcpu_ida nolock is a modified percpu_ida algorithm without
percpu_ida_cpu locks and without cross-cpu tag stealing.
It's faster than existing percpu_ida, but not as fast as pcpu_freelist.

bt is a variant of block/blk-mq-tag.c simlified and customized
for bpf use case. bt w/smp_align is using cache line for every 'long'
(similar to blk-mq-tag). bt no/smp_align allocates 'long'
bitmasks continuously to save memory. It's comparable to percpu_ida
and in some cases faster, but slower than percpu_freelist

hlist+spinlock is the simplest free list with single spinlock.
As expeceted it has very bad scaling in SMP.

kmalloc is existing implementation which is still available via
BPF_F_NO_PREALLOC flag. It's significantly slower in single cpu and
in 8 cpu setup it's 3 times slower than pre-allocation with pcpu_freelist,
but saves memory, so in cases where map->max_entries can be large
and number of map update/delete per second is low, it may make
sense to use it.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
include/linux/bpf.h | 1 +
include/uapi/linux/bpf.h | 3 +
kernel/bpf/hashtab.c | 264 ++++++++++++++++++++++++++++++++++-------------
kernel/bpf/syscall.c | 2 +-
4 files changed, 196 insertions(+), 74 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4b070827200d..c81efb10bbb5 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -37,6 +37,7 @@ struct bpf_map {
u32 key_size;
u32 value_size;
u32 max_entries;
+ u32 map_flags;
u32 pages;
struct user_struct *user;
const struct bpf_map_ops *ops;
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 6496f98d3d68..5eeb2ca9441e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -101,12 +101,15 @@ enum bpf_prog_type {
#define BPF_NOEXIST 1 /* create new element if it didn't exist */
#define BPF_EXIST 2 /* update existing element */

+#define BPF_F_NO_PREALLOC (1ULL << 0)
+
union bpf_attr {
struct { /* anonymous struct used by BPF_MAP_CREATE command */
__u32 map_type; /* one of enum bpf_map_type */
__u32 key_size; /* size of key in bytes */
__u32 value_size; /* size of value in bytes */
__u32 max_entries; /* max number of entries in a map */
+ __u32 map_flags; /* prealloc or not */
};

struct { /* anonymous struct used by BPF_MAP_*_ELEM commands */
diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index a68e95133fcd..7d5f539d196b 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1,4 +1,5 @@
/* Copyright (c) 2011-2014 PLUMgrid, http://plumgrid.com
+ * Copyright (c) 2016 Facebook
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of version 2 of the GNU General Public
@@ -13,6 +14,7 @@
#include <linux/jhash.h>
#include <linux/filter.h>
#include <linux/vmalloc.h>
+#include "percpu_freelist.h"

struct bucket {
struct hlist_head head;
@@ -22,6 +24,8 @@ struct bucket {
struct bpf_htab {
struct bpf_map map;
struct bucket *buckets;
+ void *elems;
+ struct pcpu_freelist freelist;
atomic_t count; /* number of elements in this hashtable */
u32 n_buckets; /* number of hash buckets */
u32 elem_size; /* size of each element in bytes */
@@ -29,15 +33,111 @@ struct bpf_htab {

/* each htab element is struct htab_elem + key + value */
struct htab_elem {
- struct hlist_node hash_node;
- struct rcu_head rcu;
union {
- u32 hash;
- u32 key_size;
+ struct hlist_node hash_node;
+ struct bpf_htab *htab;
+ struct pcpu_freelist_node fnode;
};
+ struct rcu_head rcu;
+ u32 hash;
char key[0] __aligned(8);
};

+static inline void htab_elem_set_ptr(struct htab_elem *l, u32 key_size,
+ void __percpu *pptr)
+{
+ *(void __percpu **)(l->key + key_size) = pptr;
+}
+
+static inline void __percpu *htab_elem_get_ptr(struct htab_elem *l, u32 key_size)
+{
+ return *(void __percpu **)(l->key + key_size);
+}
+
+static struct htab_elem *get_htab_elem(struct bpf_htab *htab, int i)
+{
+ return (struct htab_elem *) (htab->elems + i * htab->elem_size);
+}
+
+static void htab_free_elems(struct bpf_htab *htab)
+{
+ int i;
+
+ if (htab->map.map_type != BPF_MAP_TYPE_PERCPU_HASH)
+ goto free_elems;
+
+ for (i = 0; i < htab->map.max_entries; i++) {
+ void __percpu *pptr;
+
+ pptr = htab_elem_get_ptr(get_htab_elem(htab, i),
+ htab->map.key_size);
+ free_percpu(pptr);
+ }
+free_elems:
+ vfree(htab->elems);
+}
+
+/* if requested map size is larger than memlock limit, reject it early */
+static int precharge_memlock(u32 pages)
+{
+ struct user_struct *user = get_current_user();
+ unsigned long memlock_limit, cur;
+
+ memlock_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+ cur = atomic_long_read(&user->locked_vm);
+ free_uid(user);
+ if (cur + pages > memlock_limit)
+ return -EPERM;
+ return 0;
+}
+
+static int prealloc_elems_and_freelist(struct bpf_htab *htab)
+{
+ int err = -ENOMEM, i, cpu, pcpu_entries;
+
+ htab->elems = vzalloc(htab->elem_size * htab->map.max_entries);
+ if (!htab->elems)
+ return -ENOMEM;
+
+ if (htab->map.map_type != BPF_MAP_TYPE_PERCPU_HASH)
+ goto skip_percpu_elems;
+
+ for (i = 0; i < htab->map.max_entries; i++) {
+ u32 size = round_up(htab->map.value_size, 8);
+ void __percpu *pptr;
+
+ pptr = __alloc_percpu_gfp(size, 8, GFP_USER | __GFP_NOWARN);
+ if (!pptr)
+ goto free_elems;
+ htab_elem_set_ptr(get_htab_elem(htab, i), htab->map.key_size,
+ pptr);
+ }
+
+skip_percpu_elems:
+ err = pcpu_freelist_init(&htab->freelist);
+ if (err)
+ goto free_elems;
+
+ pcpu_entries = htab->map.max_entries / num_possible_cpus() + 1;
+ i = 0;
+ for_each_possible_cpu(cpu) {
+ struct htab_elem *elem;
+again:
+ elem = get_htab_elem(htab, i);
+ pcpu_freelist_push_cpu(&htab->freelist, &elem->fnode, cpu);
+ i++;
+ if (i == htab->map.max_entries)
+ break;
+ if (i % pcpu_entries)
+ goto again;
+ }
+ return 0;
+
+free_elems:
+ htab_free_elems(htab);
+ return err;
+}
+
/* Called from syscall */
static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
{
@@ -46,6 +146,10 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
int err, i;
u64 cost;

+ if (attr->map_flags & ~BPF_F_NO_PREALLOC)
+ /* reserved bits should not be used */
+ return ERR_PTR(-EINVAL);
+
htab = kzalloc(sizeof(*htab), GFP_USER);
if (!htab)
return ERR_PTR(-ENOMEM);
@@ -55,6 +159,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
htab->map.key_size = attr->key_size;
htab->map.value_size = attr->value_size;
htab->map.max_entries = attr->max_entries;
+ htab->map.map_flags = attr->map_flags;

/* check sanity of attributes.
* value_size == 0 may be allowed in the future to use map as a set
@@ -92,7 +197,7 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
if (percpu)
htab->elem_size += sizeof(void *);
else
- htab->elem_size += htab->map.value_size;
+ htab->elem_size += round_up(htab->map.value_size, 8);

/* prevent zero size kmalloc and check for u32 overflow */
if (htab->n_buckets == 0 ||
@@ -112,6 +217,10 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)

htab->map.pages = round_up(cost, PAGE_SIZE) >> PAGE_SHIFT;

+ err = precharge_memlock(htab->map.pages);
+ if (err)
+ goto free_htab;
+
err = -ENOMEM;
htab->buckets = kmalloc_array(htab->n_buckets, sizeof(struct bucket),
GFP_USER | __GFP_NOWARN);
@@ -127,10 +236,16 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
raw_spin_lock_init(&htab->buckets[i].lock);
}

- atomic_set(&htab->count, 0);
+ if (!(attr->map_flags & BPF_F_NO_PREALLOC)) {
+ err = prealloc_elems_and_freelist(htab);
+ if (err)
+ goto free_buckets;
+ }

return &htab->map;

+free_buckets:
+ kvfree(htab->buckets);
free_htab:
kfree(htab);
return ERR_PTR(err);
@@ -249,42 +364,42 @@ find_first_elem:
}
}

- /* itereated over all buckets and all elements */
+ /* iterated over all buckets and all elements */
return -ENOENT;
}

-
-static inline void htab_elem_set_ptr(struct htab_elem *l, u32 key_size,
- void __percpu *pptr)
-{
- *(void __percpu **)(l->key + key_size) = pptr;
-}
-
-static inline void __percpu *htab_elem_get_ptr(struct htab_elem *l, u32 key_size)
+static void htab_elem_free(struct bpf_htab *htab, struct htab_elem *l)
{
- return *(void __percpu **)(l->key + key_size);
-}
-
-static void htab_percpu_elem_free(struct htab_elem *l)
-{
- free_percpu(htab_elem_get_ptr(l, l->key_size));
+ if (htab->map.map_type == BPF_MAP_TYPE_PERCPU_HASH)
+ free_percpu(htab_elem_get_ptr(l, htab->map.key_size));
kfree(l);
+
}

-static void htab_percpu_elem_free_rcu(struct rcu_head *head)
+static void htab_elem_free_rcu(struct rcu_head *head)
{
struct htab_elem *l = container_of(head, struct htab_elem, rcu);
+ struct bpf_htab *htab = l->htab;

- htab_percpu_elem_free(l);
+ /* must increment bpf_prog_active to avoid kprobe+bpf triggering while
+ * we're calling kfree, otherwise deadlock is possible if kprobes
+ * are placed somewhere inside of slub
+ */
+ preempt_disable();
+ __this_cpu_inc(bpf_prog_active);
+ htab_elem_free(htab, l);
+ __this_cpu_dec(bpf_prog_active);
+ preempt_enable();
}

-static void free_htab_elem(struct htab_elem *l, bool percpu, u32 key_size)
+static void free_htab_elem(struct bpf_htab *htab, struct htab_elem *l)
{
- if (percpu) {
- l->key_size = key_size;
- call_rcu(&l->rcu, htab_percpu_elem_free_rcu);
+ if (!(htab->map.map_flags & BPF_F_NO_PREALLOC)) {
+ pcpu_freelist_push(&htab->freelist, &l->fnode);
} else {
- kfree_rcu(l, rcu);
+ atomic_dec(&htab->count);
+ l->htab = htab;
+ call_rcu(&l->rcu, htab_elem_free_rcu);
}
}

@@ -293,23 +408,39 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
bool percpu, bool onallcpus)
{
u32 size = htab->map.value_size;
+ bool prealloc = !(htab->map.map_flags & BPF_F_NO_PREALLOC);
struct htab_elem *l_new;
void __percpu *pptr;

- l_new = kmalloc(htab->elem_size, GFP_ATOMIC | __GFP_NOWARN);
- if (!l_new)
- return NULL;
+ if (prealloc) {
+ l_new = (struct htab_elem *)pcpu_freelist_pop(&htab->freelist);
+ if (!l_new)
+ return ERR_PTR(-E2BIG);
+ } else {
+ if (atomic_inc_return(&htab->count) > htab->map.max_entries) {
+ atomic_dec(&htab->count);
+ return ERR_PTR(-E2BIG);
+ }
+ l_new = kmalloc(htab->elem_size, GFP_ATOMIC | __GFP_NOWARN);
+ if (!l_new)
+ return ERR_PTR(-ENOMEM);
+ }

memcpy(l_new->key, key, key_size);
if (percpu) {
/* round up value_size to 8 bytes */
size = round_up(size, 8);

- /* alloc_percpu zero-fills */
- pptr = __alloc_percpu_gfp(size, 8, GFP_ATOMIC | __GFP_NOWARN);
- if (!pptr) {
- kfree(l_new);
- return NULL;
+ if (prealloc) {
+ pptr = htab_elem_get_ptr(l_new, key_size);
+ } else {
+ /* alloc_percpu zero-fills */
+ pptr = __alloc_percpu_gfp(size, 8,
+ GFP_ATOMIC | __GFP_NOWARN);
+ if (!pptr) {
+ kfree(l_new);
+ return ERR_PTR(-ENOMEM);
+ }
}

if (!onallcpus) {
@@ -324,7 +455,8 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
off += size;
}
}
- htab_elem_set_ptr(l_new, key_size, pptr);
+ if (!prealloc)
+ htab_elem_set_ptr(l_new, key_size, pptr);
} else {
memcpy(l_new->key + round_up(key_size, 8), value, size);
}
@@ -336,12 +468,6 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
static int check_flags(struct bpf_htab *htab, struct htab_elem *l_old,
u64 map_flags)
{
- if (!l_old && unlikely(atomic_read(&htab->count) >= htab->map.max_entries))
- /* if elem with this 'key' doesn't exist and we've reached
- * max_entries limit, fail insertion of new elem
- */
- return -E2BIG;
-
if (l_old && map_flags == BPF_NOEXIST)
/* elem already exists */
return -EEXIST;
@@ -375,13 +501,6 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,

hash = htab_map_hash(key, key_size);

- /* allocate new element outside of the lock, since
- * we're most likley going to insert it
- */
- l_new = alloc_htab_elem(htab, key, value, key_size, hash, false, false);
- if (!l_new)
- return -ENOMEM;
-
b = __select_bucket(htab, hash);
head = &b->head;

@@ -394,21 +513,24 @@ static int htab_map_update_elem(struct bpf_map *map, void *key, void *value,
if (ret)
goto err;

+ l_new = alloc_htab_elem(htab, key, value, key_size, hash, false, false);
+ if (IS_ERR(l_new)) {
+ /* all pre-allocated elements are in use or memory exhausted */
+ ret = PTR_ERR(l_new);
+ goto err;
+ }
+
/* add new element to the head of the list, so that
* concurrent search will find it before old elem
*/
hlist_add_head_rcu(&l_new->hash_node, head);
if (l_old) {
hlist_del_rcu(&l_old->hash_node);
- kfree_rcu(l_old, rcu);
- } else {
- atomic_inc(&htab->count);
+ free_htab_elem(htab, l_old);
}
- raw_spin_unlock_irqrestore(&b->lock, flags);
- return 0;
+ ret = 0;
err:
raw_spin_unlock_irqrestore(&b->lock, flags);
- kfree(l_new);
return ret;
}

@@ -466,12 +588,11 @@ static int __htab_percpu_map_update_elem(struct bpf_map *map, void *key,
} else {
l_new = alloc_htab_elem(htab, key, value, key_size,
hash, true, onallcpus);
- if (!l_new) {
- ret = -ENOMEM;
+ if (IS_ERR(l_new)) {
+ ret = PTR_ERR(l_new);
goto err;
}
hlist_add_head_rcu(&l_new->hash_node, head);
- atomic_inc(&htab->count);
}
ret = 0;
err:
@@ -489,7 +610,6 @@ static int htab_percpu_map_update_elem(struct bpf_map *map, void *key,
static int htab_map_delete_elem(struct bpf_map *map, void *key)
{
struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
- bool percpu = map->map_type == BPF_MAP_TYPE_PERCPU_HASH;
struct hlist_head *head;
struct bucket *b;
struct htab_elem *l;
@@ -511,8 +631,7 @@ static int htab_map_delete_elem(struct bpf_map *map, void *key)

if (l) {
hlist_del_rcu(&l->hash_node);
- atomic_dec(&htab->count);
- free_htab_elem(l, percpu, key_size);
+ free_htab_elem(htab, l);
ret = 0;
}

@@ -531,17 +650,10 @@ static void delete_all_elements(struct bpf_htab *htab)

hlist_for_each_entry_safe(l, n, head, hash_node) {
hlist_del_rcu(&l->hash_node);
- atomic_dec(&htab->count);
- if (htab->map.map_type == BPF_MAP_TYPE_PERCPU_HASH) {
- l->key_size = htab->map.key_size;
- htab_percpu_elem_free(l);
- } else {
- kfree(l);
- }
+ htab_elem_free(htab, l);
}
}
}
-
/* Called when map->refcnt goes to zero, either from workqueue or from syscall */
static void htab_map_free(struct bpf_map *map)
{
@@ -554,10 +666,16 @@ static void htab_map_free(struct bpf_map *map)
*/
synchronize_rcu();

- /* some of kfree_rcu() callbacks for elements of this map may not have
- * executed. It's ok. Proceed to free residual elements and map itself
+ /* some of free_htab_elem() callbacks for elements of this map may
+ * not have executed. Wait for them.
*/
- delete_all_elements(htab);
+ rcu_barrier();
+ if (htab->map.map_flags & BPF_F_NO_PREALLOC) {
+ delete_all_elements(htab);
+ } else {
+ htab_free_elems(htab);
+ pcpu_freelist_destroy(&htab->freelist);
+ }
kvfree(htab->buckets);
kfree(htab);
}
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index dc99f6a000f5..1e7dfae0de8c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -153,7 +153,7 @@ int bpf_map_new_fd(struct bpf_map *map)
offsetof(union bpf_attr, CMD##_LAST_FIELD) - \
sizeof(attr->CMD##_LAST_FIELD)) != NULL

-#define BPF_MAP_CREATE_LAST_FIELD max_entries
+#define BPF_MAP_CREATE_LAST_FIELD map_flags
/* called via syscall */
static int map_create(union bpf_attr *attr)
{
--
2.6.5