Re: [PATCH v2] bpf, docs: document BPF_MAP_TYPE_HASH and variants

[Yonghong Song <yhs@fb.com>]

On 7/15/22 6:08 AM, Donald Hunter wrote:
> Add documentation for BPF_MAP_TYPE_HASH including kernel version
> introduced, usage and examples. Document BPF_MAP_TYPE_PERCPU_HASH,
> BPF_MAP_TYPE_LRU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH variations.
> 
> Note that this file is included in the BPF documentation by the glob in
> Documentation/bpf/maps.rst
> 
> Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
> ---
>   Documentation/bpf/map_hash.rst | 181 +++++++++++++++++++++++++++++++++
>   1 file changed, 181 insertions(+)
>   create mode 100644 Documentation/bpf/map_hash.rst
> 
> diff --git a/Documentation/bpf/map_hash.rst b/Documentation/bpf/map_hash.rst
> new file mode 100644
> index 000000000000..d9e33152dae5
> --- /dev/null
> +++ b/Documentation/bpf/map_hash.rst
> @@ -0,0 +1,181 @@
> +.. SPDX-License-Identifier: GPL-2.0-only
> +.. Copyright (C) 2022 Red Hat, Inc.
> +
> +===============================================
> +BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants
> +===============================================
> +
> +.. note::
> +   - ``BPF_MAP_TYPE_HASH`` was introduced in kernel version 3.19
> +   - ``BPF_MAP_TYPE_PERCPU_HASH`` was introduced in version 4.6
> +   - Both ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> +     were introduced in version 4.10
> +
> +``BPF_MAP_TYPE_HASH`` and ``BPF_MAP_TYPE_PERCPU_HASH`` provide general
> +purpose hash map storage. Both the key and the value can be structs,
> +allowing for composite keys and values.
> +
> +The kernel is responsible for allocating and freeing key/value pairs, up
> +to the max_entries limit that you specify. Hash maps use pre-allocation
> +of hash table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be
> +used to disable pre-allocation when it is to memory expensive.
> +
> +``BPF_MAP_TYPE_PERCPU_HASH`` provides a separate value slot per
> +CPU. The per-cpu values are stored internally in an array.
> +
> +The ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> +variants add LRU semantics to their respective hash tables. An LRU hash
> +will automatically evict the least recently used entries when the hash
> +table reaches capacity. An LRU hash maintains an internal LRU list that
> +is used to select elements for eviction. This internal LRU list is
> +shared across CPUs but it is possible to request a per CPU LRU list with
> +the ``BPF_F_NO_COMMON_LRU`` flag when calling ``bpf_map_create``.
> +
> +Usage
> +=====
> +
> +.. c:function::
> +   long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
> +
> +Hash entries can be added or updated using the ``bpf_map_update_elem()``
> +helper. This helper replaces existing elements atomically. The ``flags``
> +parameter can be used to control the update behaviour:
> +
> +- ``BPF_ANY`` will create a new element or update an existing element
> +- ``BPF_NOTEXIST`` will create a new element only if one did not already
> +  exist
> +- ``BPF_EXIST`` will update an existing element
> +
> +``bpf_map_update_elem()`` returns 0 on success, or negative error in
> +case of failure.
> +
> +.. c:function::
> +   void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
> +
> +Hash entries can be retrieved using the ``bpf_map_lookup_elem()``
> +helper. This helper returns a pointer to the value associated with
> +``key``, or ``NULL`` if no entry was found.
> +
> +.. c:function::
> +   long bpf_map_delete_elem(struct bpf_map *map, const void *key)
> +
> +Hash entries can be deleted using the ``bpf_map_delete_elem()``
> +helper. This helper will return 0 on success, or negative error in case
> +of failure.
> +
> +Per CPU Hashes
> +--------------
> +
> +For ``BPF_MAP_TYPE_PERCPU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> +the ``bpf_map_update_elem()`` and ``bpf_map_lookup_elem()`` helpers
> +automatically access the hash slot for the current CPU.
> +
> +.. c:function::
> +   void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void *key, u32 cpu)
> +
> +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the
> +value in the hash slot for a specific CPU. Returns value associated with
> +``key`` on ``cpu`` , or ``NULL`` if no entry was found or ``cpu`` is
> +invalid.
> +
> +Concurrency
> +-----------
> +
> +Values stored in ``BPF_MAP_TYPE_HASH`` can be accessed concurrently by
> +programs running on different CPUs.  Since Kernel version 5.1, the BPF
> +infrastructure provides ``struct bpf_spin_lock`` to synchronize access.
> +See ``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
> +
> +Userspace
> +---------
> +
> +.. c:function::
> +   int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key)
> +
> +In userspace, is possible to iterate through the keys of a hash using

'is possible' -> 'it is possible'

> +the ``bpf_map_get_next_key()`` function. The first key can be fetched by
> +calling ``bpf_map_get_next_key()`` with ``cur_key`` set to
> +``NULL``. Subsequent calls will fetch the next key that follows the
> +current key. ``bpf_map_get_next_key()`` returns 0 on success, -ENOENT if
> +cur_key is the last key in the hash, or negative error in case of
> +failure.

There are some potential issues related to bpf_map_get_next_key() where
if it happened the *cur_key* in bpf_map_get_next_key() is deleted, the 
returned next_key will be the *first* key in the hash table. This is
an undesired behavior. So we should mention this and recommend users
to use batch-based lookup if their setup involves key deletion 
intermixed with bpf_map_get_next_key(). The details can be found here:
   https://lore.kernel.org/bpf/20200115184308.162644-6-brianvv@google.com

> +
> +Examples
> +========
> +
> +Please see the ``tools/testing/selftests/bpf`` directory for functional
> +examples.  The sample code below demonstrates API usage.
> +
[...]