Re: [PATCH v4 bpf-next 5/9] bpf: add batch ops to all htab bpf map

[Yonghong Song]

On 1/14/20 8:46 AM, Brian Vazquez wrote:
> From: Yonghong Song <yhs@xxxxxx>
> 
> htab can't use generic batch support due some problematic behaviours
> inherent to the data structre, i.e. while iterating the bpf map  a
> concurrent program might delete the next entry that batch was about to
> use, in that case there's no easy solution to retrieve the next entry,
> the issue has been discussed multiple times (see [1] and [2]).
> 
> The only way hmap can be traversed without the problem previously
> exposed is by making sure that the map is traversing entire buckets.
> This commit implements those strict requirements for hmap, the
> implementation follows the same interaction that generic support with
> some exceptions:
> 
>   - If keys/values buffer are not big enough to traverse a bucket,
>     ENOSPC will be returned.
>   - out_batch contains the value of the next bucket in the iteration, not
>     the next key, but this is transparent for the user since the user
>     should never use out_batch for other than bpf batch syscalls.
> 
> This commits implements BPF_MAP_LOOKUP_BATCH and adds support for new
> command BPF_MAP_LOOKUP_AND_DELETE_BATCH. Note that for update/delete
> batch ops it is possible to use the generic implementations.
> 
> [1] https://lore.kernel.org/bpf/20190724165803.87470-1-brianvv@xxxxxxxxxx/
> [2] https://lore.kernel.org/bpf/20190906225434.3635421-1-yhs@xxxxxx/
> 
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> Signed-off-by: Brian Vazquez <brianvv@xxxxxxxxxx>
> ---
>   include/linux/bpf.h      |   3 +
>   include/uapi/linux/bpf.h |   1 +
>   kernel/bpf/hashtab.c     | 258 +++++++++++++++++++++++++++++++++++++++
>   kernel/bpf/syscall.c     |   9 +-
>   4 files changed, 270 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 05466ad6cf1c5..3517e32149a4f 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -46,6 +46,9 @@ struct bpf_map_ops {
>   	void *(*map_lookup_elem_sys_only)(struct bpf_map *map, void *key);
>   	int (*map_lookup_batch)(struct bpf_map *map, const union bpf_attr *attr,
>   				union bpf_attr __user *uattr);
> +	int (*map_lookup_and_delete_batch)(struct bpf_map *map,
> +					   const union bpf_attr *attr,
> +					   union bpf_attr __user *uattr);
>   	int (*map_update_batch)(struct bpf_map *map, const union bpf_attr *attr,
>   				union bpf_attr __user *uattr);
>   	int (*map_delete_batch)(struct bpf_map *map, const union bpf_attr *attr,
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index e8df9ca680e0c..9536729a03d57 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -108,6 +108,7 @@ enum bpf_cmd {
>   	BPF_MAP_FREEZE,
>   	BPF_BTF_GET_NEXT_ID,
>   	BPF_MAP_LOOKUP_BATCH,
> +	BPF_MAP_LOOKUP_AND_DELETE_BATCH,
>   	BPF_MAP_UPDATE_BATCH,
>   	BPF_MAP_DELETE_BATCH,
>   };
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 22066a62c8c97..d9888acfd632b 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -17,6 +17,16 @@
>   	(BPF_F_NO_PREALLOC | BPF_F_NO_COMMON_LRU | BPF_F_NUMA_NODE |	\
>   	 BPF_F_ACCESS_MASK | BPF_F_ZERO_SEED)
>   
> +#define BATCH_OPS(_name)			\
> +	.map_lookup_batch =			\
> +	_name##_map_lookup_batch,		\
> +	.map_lookup_and_delete_batch =		\
> +	_name##_map_lookup_and_delete_batch,	\
> +	.map_update_batch =			\
> +	generic_map_update_batch,		\
> +	.map_delete_batch =			\
> +	generic_map_delete_batch
> +
>   struct bucket {
>   	struct hlist_nulls_head head;
>   	raw_spinlock_t lock;
> @@ -1232,6 +1242,250 @@ static void htab_map_seq_show_elem(struct bpf_map *map, void *key,
>   	rcu_read_unlock();
>   }
>   
> +static int
> +__htab_map_lookup_and_delete_batch(struct bpf_map *map,
> +				   const union bpf_attr *attr,
> +				   union bpf_attr __user *uattr,
> +				   bool do_delete, bool is_lru_map,
> +				   bool is_percpu)
> +{
> +	struct bpf_htab *htab = container_of(map, struct bpf_htab, map);
> +	u32 bucket_cnt, total, key_size, value_size, roundup_key_size;
> +	void *keys = NULL, *values = NULL, *value, *dst_key, *dst_val;
> +	void __user *uvalues = u64_to_user_ptr(attr->batch.values);
> +	void __user *ukeys = u64_to_user_ptr(attr->batch.keys);
> +	void *ubatch = u64_to_user_ptr(attr->batch.in_batch);
> +	u32 batch, max_count, size, bucket_size;
> +	u64 elem_map_flags, map_flags;
> +	struct hlist_nulls_head *head;
> +	struct hlist_nulls_node *n;
> +	unsigned long flags;
> +	struct htab_elem *l;
> +	struct bucket *b;
> +	int ret = 0;
> +
> +	elem_map_flags = attr->batch.elem_flags;
> +	if ((elem_map_flags & ~BPF_F_LOCK) ||
> +	    ((elem_map_flags & BPF_F_LOCK) && !map_value_has_spin_lock(map)))
> +		return -EINVAL;
> +
> +	map_flags = attr->batch.flags;
> +	if (map_flags)
> +		return -EINVAL;
> +
> +	max_count = attr->batch.count;
> +	if (!max_count)
> +		return 0;
> +
> +	batch = 0;
> +	if (ubatch && copy_from_user(&batch, ubatch, sizeof(batch)))
> +		return -EFAULT;
> +
> +	if (batch >= htab->n_buckets)
> +		return -ENOENT;
> +
> +	key_size = htab->map.key_size;
> +	roundup_key_size = round_up(htab->map.key_size, 8);
> +	value_size = htab->map.value_size;
> +	size = round_up(value_size, 8);
> +	if (is_percpu)
> +		value_size = size * num_possible_cpus();
> +	total = 0;
> +	bucket_size = 1;

Have you checked typical hash table linklist length?
Maybe initial value bucket_size = 2 is able to cover most common cases?

> +
> +alloc:
> +	/* We cannot do copy_from_user or copy_to_user inside
> +	 * the rcu_read_lock. Allocate enough space here.
> +	 */
> +	keys = kvmalloc(key_size * bucket_size, GFP_USER | __GFP_NOWARN);
> +	values = kvmalloc(value_size * bucket_size, GFP_USER | __GFP_NOWARN);
> +	if (!keys || !values) {
> +		ret = -ENOMEM;
> +		goto out;

In this case, we won't copy batch and total to user buffer. Maybe we 
should do that?


> +	}
> +
> +again:
> +	preempt_disable();
> +	this_cpu_inc(bpf_prog_active);
> +	rcu_read_lock();
> +again_nocopy:
> +	dst_key = keys;
> +	dst_val = values;
> +	b = &htab->buckets[batch];
> +	head = &b->head;
> +	raw_spin_lock_irqsave(&b->lock, flags);
> +
> +	bucket_cnt = 0;
> +	hlist_nulls_for_each_entry_rcu(l, n, head, hash_node)
> +		bucket_cnt++;
> +
> +	if (bucket_cnt > (max_count - total)) {
> +		if (total == 0)
> +			ret = -ENOSPC;
> +		raw_spin_unlock_irqrestore(&b->lock, flags);
> +		rcu_read_unlock();
> +		this_cpu_dec(bpf_prog_active);
> +		preempt_enable();
> +		goto after_loop;
> +	}
> +
> +	if (bucket_cnt > bucket_size) {
> +		bucket_size = bucket_cnt;
> +		raw_spin_unlock_irqrestore(&b->lock, flags);
> +		rcu_read_unlock();
> +		this_cpu_dec(bpf_prog_active);
> +		preempt_enable();
> +		kvfree(keys);
> +		kvfree(values);
> +		goto alloc;
> +	}
> +
> +	hlist_nulls_for_each_entry_safe(l, n, head, hash_node) {
> +		memcpy(dst_key, l->key, key_size);
> +
> +		if (is_percpu) {
> +			int off = 0, cpu;
> +			void __percpu *pptr;
> +
> +			pptr = htab_elem_get_ptr(l, map->key_size);
> +			for_each_possible_cpu(cpu) {
> +				bpf_long_memcpy(dst_val + off,
> +						per_cpu_ptr(pptr, cpu), size);
> +				off += size;
> +			}
> +		} else {
> +			value = l->key + roundup_key_size;
> +			if (elem_map_flags & BPF_F_LOCK)
> +				copy_map_value_locked(map, dst_val, value,
> +						      true);
> +			else
> +				copy_map_value(map, dst_val, value);
> +			check_and_init_map_lock(map, dst_val);
> +		}
> +		if (do_delete) {
> +			hlist_nulls_del_rcu(&l->hash_node);
> +			if (is_lru_map)
> +				bpf_lru_push_free(&htab->lru, &l->lru_node);
> +			else
> +				free_htab_elem(htab, l);
> +		}
> +		dst_key += key_size;
> +		dst_val += value_size;
> +	}
> +
> +	raw_spin_unlock_irqrestore(&b->lock, flags);
> +	/* If we are not copying data, we can go to next bucket and avoid
> +	 * unlocking the rcu.
> +	 */
> +	if (!bucket_cnt && (batch + 1 < htab->n_buckets)) {
> +		batch++;
> +		goto again_nocopy;
> +	}
> +
> +	rcu_read_unlock();
> +	this_cpu_dec(bpf_prog_active);
> +	preempt_enable();
> +	if (bucket_cnt && (copy_to_user(ukeys + total * key_size, keys,
> +	    key_size * bucket_cnt) ||
> +	    copy_to_user(uvalues + total * value_size, values,
> +	    value_size * bucket_cnt))) {
> +		ret = -EFAULT;
> +		goto after_loop;
> +	}
> +
> +	total += bucket_cnt;
> +	batch++;
> +	if (batch >= htab->n_buckets) {
> +		ret = -ENOENT;
> +		goto after_loop;
> +	}
> +	goto again;
> +
> +after_loop:
> +	if (ret && (ret != -ENOENT && ret != -EFAULT))
> +		goto out;

We won't have many error codes reaching here, -ENOENT, -EFAULT, -ENOSPC,
and possibly -ENOMEM.
Maybe just
	if (ret == -EFAULT)
		goto out;

> +
> +	/* copy # of entries and next batch */
> +	ubatch = u64_to_user_ptr(attr->batch.out_batch);
> +	if (copy_to_user(ubatch, &batch, sizeof(batch)) ||
> +	    put_user(total, &uattr->batch.count))
> +		ret = -EFAULT;
> +
> +out:
> +	kvfree(keys);
> +	kvfree(values);
> +	return ret;
> +}
> +
[...]