[PATCH bpf-next v3 7/7] docs/bpf: Add documentation for map type BPF_MAP_TYPE_CGRP_STROAGE

Yonghong Song
2022-10-21T23:45:10Z

Add some descriptions and examples for BPF_MAP_TYPE_CGRP_STROAGE.
Also illustate the major difference between BPF_MAP_TYPE_CGRP_STROAGE
and BPF_MAP_TYPE_CGROUP_STORAGE and recommend to use
BPF_MAP_TYPE_CGRP_STROAGE instead of BPF_MAP_TYPE_CGROUP_STORAGE
in the end.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
Documentation/bpf/map_cgrp_storage.rst | 109 +++++++++++++++++++++++++
1 file changed, 109 insertions(+)
create mode 100644 Documentation/bpf/map_cgrp_storage.rst

diff --git a/Documentation/bpf/map_cgrp_storage.rst b/Documentation/bpf/map_cgrp_storage.rst
new file mode 100644
index 000000000000..ebccc96564f7
--- /dev/null
+++ b/Documentation/bpf/map_cgrp_storage.rst
@@ -0,0 +1,109 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Meta Platforms, Inc. and affiliates.
+
+===========================
+BPF_MAP_TYPE_CGRP_STORAGE
+===========================
+
+The ``BPF_MAP_TYPE_CGRP_STORAGE`` map type represents a local fix-sized
+storage for cgroups. It is only available with ``CONFIG_CGROUP_BPF``.
+The programs are made available by the same Kconfig. The
+data for a particular cgroup can be retrieved by looking up the map
+with that cgroup.
+
+This document describes the usage and semantics of the
+``BPF_MAP_TYPE_CGRP_STORAGE`` map type.
+
+Usage
+=====
+
+The map key must be ``sizeof(int)`` representing a cgroup fd.
+To access the storage in a program, use ``bpf_cgrp_storage_get``::
+
+ void *bpf_cgrp_storage_get(struct bpf_map *map, struct cgroup *cgroup, void *value, u64 flags)
+
+``flags`` could be 0 or ``BPF_LOCAL_STORAGE_GET_F_CREATE`` which indicates that
+a new local storage will be created if one does not exist.
+
+The local storage can be removed with ``bpf_cgrp_storage_delete``::
+
+ long bpf_cgrp_storage_delete(struct bpf_map *map, struct cgroup *cgroup)
+
+The map is available to all program types.
+
+Examples
+========
+
+A bpf program example with BPF_MAP_TYPE_CGRP_STORAGE::
+
+ #include <vmlinux.h>
+ #include <bpf/bpf_helpers.h>
+ #include <bpf/bpf_tracing.h>
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_CGRP_STORAGE);
+ __uint(map_flags, BPF_F_NO_PREALLOC);
+ __type(key, int);
+ __type(value, long);
+ } cgrp_storage SEC(".maps");
+
+ SEC("tp_btf/sys_enter")
+ int BPF_PROG(on_enter, struct pt_regs *regs, long id)
+ {
+ struct task_struct *task = bpf_get_current_task_btf();
+ long *ptr;
+
+ ptr = bpf_cgrp_storage_get(&cgrp_storage, task->cgroups->dfl_cgrp, 0,
+ BPF_LOCAL_STORAGE_GET_F_CREATE);
+ if (ptr)
+ __sync_fetch_and_add(ptr, 1);
+
+ return 0;
+ }
+
+Userspace accessing map declared above::
+
+ #include <linux/bpf.h>
+ #include <linux/libbpf.h>
+
+ __u32 map_lookup(struct bpf_map *map, int cgrp_fd)
+ {
+ __u32 *value;
+ value = bpf_map_lookup_elem(bpf_map__fd(map), &cgrp_fd);
+ if (value)
+ return *value;
+ return 0;
+ }
+
+Difference Between BPF_MAP_TYPE_CGRP_STORAGE and BPF_MAP_TYPE_CGROUP_STORAGE
+============================================================================
+
+The old cgroup storage map ``BPF_MAP_TYPE_CGROUP_STORAGE`` has been marked as
+deprecated (renamed to ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``). The new
+``BPF_MAP_TYPE_CGRP_STORAGE`` map should be used instead. The following
+illusates the main difference between ``BPF_MAP_TYPE_CGRP_STORAGE`` and
+``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``.
+
+(1). ``BPF_MAP_TYPE_CGRP_STORAGE`` can be used by all program types while
+ ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` is available only to cgroup program types
+ like BPF_CGROUP_INET_INGRESS or BPF_CGROUP_SOCK_OPS, etc.
+
+(2). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports local storage for more than one
+ cgroup while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only support one cgroup
+ which is attached by a bpf program.
+
+(3). ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` allocates local storage at attach time so
+ ``bpf_get_local_storage()`` always returns non-NULL local storage.
+ ``BPF_MAP_TYPE_CGRP_STORAGE`` allocates local storage at runtime so
+ it is possible that ``bpf_cgrp_storage_get()`` may return null local storage.
+ To avoid such null local storage issue, user space can do
+ ``bpf_map_update_elem()`` to pre-allocate local storage before a bpf program
+ is attached.
+
+(4). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports deleting local storage by a bpf program
+ while ``BPF_MAP_TYPE_CGROUP_STORAGE`` only deletes storage during
+ prog detach time.
+
+So overall, ``BPF_MAP_TYPE_CGRP_STORAGE`` supports all ``BPF_MAP_TYPE_CGROUP_STORAGE``
+functionality and beyond. It is recommended to use ``BPF_MAP_TYPE_CGRP_STORAGE``
+instead of ``BPF_MAP_TYPE_CGROUP_STORAGE``.
--
2.30.2