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SubjectRe: [PATCH v3 bpf-next 02/11] bpf: add generic support for lookup and lookup_and_delete batch ops
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On 1/6/20 10:50 PM, Brian Vazquez wrote:
> On Fri, Dec 13, 2019 at 11:26 AM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 12/11/19 2:33 PM, Brian Vazquez wrote:
>>> This commit introduces generic support for the bpf_map_lookup_batch and
>>> bpf_map_lookup_and_delete_batch ops. This implementation can be used by
>>> almost all the bpf maps since its core implementation is relying on the
>>> existing map_get_next_key, map_lookup_elem and map_delete_elem
>>> functions. The bpf syscall subcommands introduced are:
>>>
>>> BPF_MAP_LOOKUP_BATCH
>>> BPF_MAP_LOOKUP_AND_DELETE_BATCH
>>>
>>> The UAPI attribute is:
>>>
>>> struct { /* struct used by BPF_MAP_*_BATCH commands */
>>> __aligned_u64 in_batch; /* start batch,
>>> * NULL to start from beginning
>>> */
>>> __aligned_u64 out_batch; /* output: next start batch */
>>> __aligned_u64 keys;
>>> __aligned_u64 values;
>>> __u32 count; /* input/output:
>>> * input: # of key/value
>>> * elements
>>> * output: # of filled elements
>>> */
>>> __u32 map_fd;
>>> __u64 elem_flags;
>>> __u64 flags;
>>> } batch;
>>>
>>> in_batch/out_batch are opaque values use to communicate between
>>> user/kernel space, in_batch/out_batch must be of key_size length.
>>>
>>> To start iterating from the beginning in_batch must be null,
>>> count is the # of key/value elements to retrieve. Note that the 'keys'
>>> buffer must be a buffer of key_size * count size and the 'values' buffer
>>> must be value_size * count, where value_size must be aligned to 8 bytes
>>> by userspace if it's dealing with percpu maps. 'count' will contain the
>>> number of keys/values successfully retrieved. Note that 'count' is an
>>> input/output variable and it can contain a lower value after a call.
>>>
>>> If there's no more entries to retrieve, ENOENT will be returned. If error
>>> is ENOENT, count might be > 0 in case it copied some values but there were
>>> no more entries to retrieve.
>>>
>>> Note that if the return code is an error and not -EFAULT,
>>> count indicates the number of elements successfully processed.
>>>
>>> Suggested-by: Stanislav Fomichev <sdf@google.com>
>>> Signed-off-by: Brian Vazquez <brianvv@google.com>
>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>> ---
>>> include/linux/bpf.h | 11 +++
>>> include/uapi/linux/bpf.h | 19 +++++
>>> kernel/bpf/syscall.c | 172 +++++++++++++++++++++++++++++++++++++++
>>> 3 files changed, 202 insertions(+)
>> [...]
>>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>>> index 2530266fa6477..708aa89fe2308 100644
>>> --- a/kernel/bpf/syscall.c
>>> +++ b/kernel/bpf/syscall.c
>>> @@ -1206,6 +1206,120 @@ static int map_get_next_key(union bpf_attr *attr)
>>> return err;
>>> }
>>>
>>> +#define MAP_LOOKUP_RETRIES 3
>>> +
>>> +static int __generic_map_lookup_batch(struct bpf_map *map,
>>> + const union bpf_attr *attr,
>>> + union bpf_attr __user *uattr,
>>> + bool do_delete)
>>> +{
>>> + void __user *ubatch = u64_to_user_ptr(attr->batch.in_batch);
>>> + void __user *uobatch = u64_to_user_ptr(attr->batch.out_batch);
>>> + void __user *values = u64_to_user_ptr(attr->batch.values);
>>> + void __user *keys = u64_to_user_ptr(attr->batch.keys);
>>> + void *buf, *prev_key, *key, *value;
>>> + u32 value_size, cp, max_count;
>>> + bool first_key = false;
>>> + int err, retry = MAP_LOOKUP_RETRIES;
>>
>> Could you try to use reverse Christmas tree style declaration here?
>
> ACK
>>
>>> +
>>> + if (attr->batch.elem_flags & ~BPF_F_LOCK)
>>> + return -EINVAL;
>>> +
>>> + if ((attr->batch.elem_flags & BPF_F_LOCK) &&
>>> + !map_value_has_spin_lock(map))
>>> + return -EINVAL;
>>> +
>>> + value_size = bpf_map_value_size(map);
>>> +
>>> + max_count = attr->batch.count;
>>> + if (!max_count)
>>> + return 0;
>>> +
>>> + buf = kmalloc(map->key_size + value_size, GFP_USER | __GFP_NOWARN);
>>> + if (!buf)
>>> + return -ENOMEM;
>>> +
>>> + err = -EFAULT;
>>> + first_key = false;
>>> + if (ubatch && copy_from_user(buf, ubatch, map->key_size))
>>> + goto free_buf;
>>> + key = buf;
>>> + value = key + map->key_size;
>>> + if (!ubatch) {
>>> + prev_key = NULL;
>>> + first_key = true;
>>> + }
>>> +
>>> + for (cp = 0; cp < max_count;) {
>>> + if (cp || first_key) {
>>> + rcu_read_lock();
>>> + err = map->ops->map_get_next_key(map, prev_key, key);
>>> + rcu_read_unlock();
>>> + if (err)
>>> + break;
>>> + }
>>> + err = bpf_map_copy_value(map, key, value,
>>> + attr->batch.elem_flags, do_delete);
>>> +
>>> + if (err == -ENOENT) {
>>> + if (retry) {
>>> + retry--;
>>> + continue;
>>> + }
>>> + err = -EINTR;
>>> + break;
>>> + }
>>> +
>>> + if (err)
>>> + goto free_buf;
>>> +
>>> + if (copy_to_user(keys + cp * map->key_size, key,
>>> + map->key_size)) {
>>> + err = -EFAULT;
>>> + goto free_buf;
>>> + }
>>> + if (copy_to_user(values + cp * value_size, value, value_size)) {
>>> + err = -EFAULT;
>>> + goto free_buf;
>>> + }
>>> +
>>> + prev_key = key;
>>> + retry = MAP_LOOKUP_RETRIES;
>>> + cp++;
>>> + }
>>> +
>>> + if (!err) {
>>> + rcu_read_lock();
>>> + err = map->ops->map_get_next_key(map, prev_key, key);
>>> + rcu_read_unlock();
>>> + }
>>> +
>>> + if (err)
>>> + memset(key, 0, map->key_size);
>>
>> So if any error happens due to above map_get_next_key() or earlier
>> error, the next "batch" returned to user could be "0". What should
>> user space handle this? Ultimately, the user space needs to start
>> from the beginning again?
>>
>> What I mean is here how we could design an interface so user
>> space, if no -EFAULT error, can successfully get all elements
>> without duplication.
>>
>> One way to do here is just return -EFAULT if we cannot get
>> proper next key. But maybe we could have better mechanism
>> when we try to implement what user space codes will look like.
>
> I was thinking that instead of using the "next key" as a token we
> could use the last value successfully copied as the token, that way
> user space code would always be able to start/retry from the last
> processed entry. Do you think this would work?

Yes, this should work.


>>
>>> +
>>> + if ((copy_to_user(&uattr->batch.count, &cp, sizeof(cp)) ||
>>> + (copy_to_user(uobatch, key, map->key_size))))
>>> + err = -EFAULT;
>>> +
>>> +free_buf:
>>> + kfree(buf);
>>> + return err;
>>> +}
>>> +
>> [...]
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