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![Yonghong Song](/assets/img/avatar_default.png)
Add some descriptions and examples for BPF_MAP_TYPE_CGRP_STORAGE. Also illustate the major difference between BPF_MAP_TYPE_CGRP_STORAGE and BPF_MAP_TYPE_CGROUP_STORAGE and recommend to use BPF_MAP_TYPE_CGRP_STORAGE instead of BPF_MAP_TYPE_CGROUP_STORAGE in the end. Acked-by: David Vernet <void@manifault.com> Signed-off-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/r/20221026042922.676383-1-yhs@fb.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
109 lines
4 KiB
ReStructuredText
109 lines
4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0-only
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.. Copyright (C) 2022 Meta Platforms, Inc. and affiliates.
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=========================
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BPF_MAP_TYPE_CGRP_STORAGE
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=========================
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The ``BPF_MAP_TYPE_CGRP_STORAGE`` map type represents a local fix-sized
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storage for cgroups. It is only available with ``CONFIG_CGROUPS``.
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The programs are made available by the same Kconfig. The
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data for a particular cgroup can be retrieved by looking up the map
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with that cgroup.
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This document describes the usage and semantics of the
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``BPF_MAP_TYPE_CGRP_STORAGE`` map type.
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Usage
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=====
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The map key must be ``sizeof(int)`` representing a cgroup fd.
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To access the storage in a program, use ``bpf_cgrp_storage_get``::
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void *bpf_cgrp_storage_get(struct bpf_map *map, struct cgroup *cgroup, void *value, u64 flags)
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``flags`` could be 0 or ``BPF_LOCAL_STORAGE_GET_F_CREATE`` which indicates that
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a new local storage will be created if one does not exist.
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The local storage can be removed with ``bpf_cgrp_storage_delete``::
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long bpf_cgrp_storage_delete(struct bpf_map *map, struct cgroup *cgroup)
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The map is available to all program types.
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Examples
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========
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A BPF program example with BPF_MAP_TYPE_CGRP_STORAGE::
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#include <vmlinux.h>
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#include <bpf/bpf_helpers.h>
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#include <bpf/bpf_tracing.h>
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struct {
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__uint(type, BPF_MAP_TYPE_CGRP_STORAGE);
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__uint(map_flags, BPF_F_NO_PREALLOC);
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__type(key, int);
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__type(value, long);
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} cgrp_storage SEC(".maps");
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SEC("tp_btf/sys_enter")
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int BPF_PROG(on_enter, struct pt_regs *regs, long id)
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{
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struct task_struct *task = bpf_get_current_task_btf();
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long *ptr;
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ptr = bpf_cgrp_storage_get(&cgrp_storage, task->cgroups->dfl_cgrp, 0,
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BPF_LOCAL_STORAGE_GET_F_CREATE);
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if (ptr)
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__sync_fetch_and_add(ptr, 1);
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return 0;
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}
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Userspace accessing map declared above::
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#include <linux/bpf.h>
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#include <linux/libbpf.h>
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__u32 map_lookup(struct bpf_map *map, int cgrp_fd)
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{
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__u32 *value;
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value = bpf_map_lookup_elem(bpf_map__fd(map), &cgrp_fd);
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if (value)
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return *value;
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return 0;
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}
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Difference Between BPF_MAP_TYPE_CGRP_STORAGE and BPF_MAP_TYPE_CGROUP_STORAGE
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============================================================================
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The old cgroup storage map ``BPF_MAP_TYPE_CGROUP_STORAGE`` has been marked as
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deprecated (renamed to ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``). The new
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``BPF_MAP_TYPE_CGRP_STORAGE`` map should be used instead. The following
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illusates the main difference between ``BPF_MAP_TYPE_CGRP_STORAGE`` and
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``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``.
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(1). ``BPF_MAP_TYPE_CGRP_STORAGE`` can be used by all program types while
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``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` is available only to cgroup program types
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like BPF_CGROUP_INET_INGRESS or BPF_CGROUP_SOCK_OPS, etc.
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(2). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports local storage for more than one
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cgroup while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only supports one cgroup
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which is attached by a BPF program.
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(3). ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` allocates local storage at attach time so
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``bpf_get_local_storage()`` always returns non-NULL local storage.
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``BPF_MAP_TYPE_CGRP_STORAGE`` allocates local storage at runtime so
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it is possible that ``bpf_cgrp_storage_get()`` may return null local storage.
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To avoid such null local storage issue, user space can do
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``bpf_map_update_elem()`` to pre-allocate local storage before a BPF program
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is attached.
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(4). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports deleting local storage by a BPF program
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while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only deletes storage during
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prog detach time.
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So overall, ``BPF_MAP_TYPE_CGRP_STORAGE`` supports all ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``
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functionality and beyond. It is recommended to use ``BPF_MAP_TYPE_CGRP_STORAGE``
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instead of ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``.
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