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Date: Tue, 7 Jan 2020 18:32:58 -0600
From: Brian Vazquez <brianvv.kernel@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Brian Vazquez <brianvv@...gle.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
"David S . Miller" <davem@...emloft.net>,
Stanislav Fomichev <sdf@...gle.com>,
Petar Penkov <ppenkov@...gle.com>,
Willem de Bruijn <willemb@...gle.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next 06/11] bpf: add batch ops to all htab bpf map
On Tue, Jan 7, 2020 at 12:18 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 1/6/20 11:02 PM, Brian Vazquez wrote:
> > On Fri, Dec 13, 2019 at 12:58 PM Yonghong Song <yhs@...com> wrote:
> >>
> >>
> >>
> >> On 12/11/19 2:33 PM, Brian Vazquez wrote:
> >>> From: Yonghong Song <yhs@...com>
> >>>
> >>> 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.
> >>>
> >>> Note that only lookup and lookup_and_delete batch ops require the hmap
> >>> specific implementation, update/delete batch ops can be the generic
> >>> ones.
> >>>
> >>> [1] https://lore.kernel.org/bpf/20190724165803.87470-1-brianvv@google.com/
> >>> [2] https://lore.kernel.org/bpf/20190906225434.3635421-1-yhs@fb.com/
> >>>
> >>> Signed-off-by: Yonghong Song <yhs@...com>
> >>> Signed-off-by: Brian Vazquez <brianvv@...gle.com>
> >>> ---
> >>> kernel/bpf/hashtab.c | 242 +++++++++++++++++++++++++++++++++++++++++++
> >>> 1 file changed, 242 insertions(+)
> >>>
> >>> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> >>> index 22066a62c8c97..fac107bdaf9ec 100644
> >>> --- a/kernel/bpf/hashtab.c
> >>> +++ b/kernel/bpf/hashtab.c
> >>> @@ -17,6 +17,17 @@
> >>> (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 +1243,233 @@ 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);
> >>> + u64 elem_map_flags, map_flags;
> >>> + struct hlist_nulls_head *head;
> >>> + u32 batch, max_count, size;
> >>> + struct hlist_nulls_node *n;
> >>> + unsigned long flags;
> >>> + struct htab_elem *l;
> >>> + struct bucket *b;
> >>> + int ret = 0;
> >>> +
> >>> + max_count = attr->batch.count;
> >>> + if (!max_count)
> >>> + return 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;
> >>> +
> >>> + batch = 0;
> >>> + if (ubatch && copy_from_user(&batch, ubatch, sizeof(batch)))
> >>> + return -EFAULT;
> >>> +
> >>> + if (batch >= htab->n_buckets)
> >>> + return -ENOENT;
> >>> +
> >>> + /* We cannot do copy_from_user or copy_to_user inside
> >>> + * the rcu_read_lock. Allocate enough space here.
> >>> + */
> >>> + 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();
> >>> + keys = kvmalloc(key_size, GFP_USER | __GFP_NOWARN);
> >>> + values = kvmalloc(value_size, GFP_USER | __GFP_NOWARN);
> >>> + if (!keys || !values) {
> >>> + ret = -ENOMEM;
> >>> + goto out;
> >>> + }
> >>> +
> >>> + dst_key = keys;
> >>> + dst_val = values;
> >>> + total = 0;
> >>> +
> >>> + preempt_disable();
> >>> + this_cpu_inc(bpf_prog_active);
> >>> + rcu_read_lock();
> >>> +
> >>> +again:
> >>> + 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;
> >>> + goto after_loop;
> >>> + }
> >>> +
> >>> + 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);
> >>> + }
> >>> + if (copy_to_user(ukeys + total * key_size, keys, key_size) ||
> >>> + copy_to_user(uvalues + total * value_size, values,
> >>> + value_size)) {
> >>
> >> We cannot do copy_to_user inside atomic region where irq is disabled
> >> with raw_spin_lock_irqsave(). We could do the following:
> >> . we kalloc memory before preempt_disable() with the current count
> >> of bucket size.
> >> . inside the raw_spin_lock_irqsave() region, we can do copy to kernel
> >> memory.
> >> . inside the raw_spin_lock_irqsave() region, if the bucket size
> >> changes, we can have a few retries to increase allocation size
> >> before giving up.
> >> Do you think this may work?
> >
> > Yes, it does.
> >
> > What should be the initial value for the allocated memory
> > max_entries/2? Do you see any issue if we just kalloc the entire
> > buffer?
>
> Allocating max_entries/2 or entire buffer risks allocating too much
> memory from the system, which may not be a good thing in a production
> system. That is why I proposed to allocate memory at bucket level.
> For a reasonable balanced hash table, this should not cause large
> memory pressure on the host. What do you think?
Sounds reasonable, I'll do that! Thanks for the feedback!
>
> >
> >>
> >>> + ret = -EFAULT;
> >>> + goto after_loop;
> >>> + }
> >>> + total++;
> >>> + }
> >>> +
> >>> + batch++;
> >>> + if (batch >= htab->n_buckets) {
> >>> + ret = -ENOENT;
> >>> + goto after_loop;
> >>> + }
> >>> +
> >>> + raw_spin_unlock_irqrestore(&b->lock, flags);
> >>> + goto again;
> >>> +
> >>> +after_loop:
> >>> + raw_spin_unlock_irqrestore(&b->lock, flags);
> >>> +
> >>> + rcu_read_unlock();
> >>> + this_cpu_dec(bpf_prog_active);
> >>> + preempt_enable();
> >>> +
> >>> + if (ret && ret != -ENOENT)
> >>> + goto out;
> >>> +
> >>> + /* copy data back to user */
> >>> + 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;
> >>> +}
> >>> +
> >> [...]
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