Re: [PATCH bpf-next v4 4/5] libbpf: Add auto-pinning of maps when loading BPF objects

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Oct 29, 2019 at 12:39 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@redhat.com>
>
> This adds support to libbpf for setting map pinning information as part of
> the BTF map declaration, to get automatic map pinning (and reuse) on load.
> The pinning type currently only supports a single PIN_BY_NAME mode, where
> each map will be pinned by its name in a path that can be overridden, but
> defaults to /sys/fs/bpf.
>
> Since auto-pinning only does something if any maps actually have a
> 'pinning' BTF attribute set, we default the new option to enabled, on the
> assumption that seamless pinning is what most callers want.
>
> When a map has a pin_path set at load time, libbpf will compare the map
> pinned at that location (if any), and if the attributes match, will re-use
> that map instead of creating a new one. If no existing map is found, the
> newly created map will instead be pinned at the location.
>
> Programs wanting to customise the pinning can override the pinning paths
> using bpf_map__set_pin_path() before calling bpf_object__load() (including
> setting it to NULL to disable pinning of a particular map).
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---

Please fix unconditional pin_path setting, with that:

Acked-by: Andrii Nakryiko <andriin@fb.com>

>  tools/lib/bpf/bpf_helpers.h |    6 ++
>  tools/lib/bpf/libbpf.c      |  144 +++++++++++++++++++++++++++++++++++++++++--
>  tools/lib/bpf/libbpf.h      |   13 ++++
>  3 files changed, 154 insertions(+), 9 deletions(-)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 2203595f38c3..0c7d28292898 100644
> --- a/tools/lib/bpf/bpf_helpers.h
> +++ b/tools/lib/bpf/bpf_helpers.h
> @@ -38,4 +38,10 @@ struct bpf_map_def {
>         unsigned int map_flags;
>  };
>

[...]

> @@ -1270,6 +1292,28 @@ static int bpf_object__init_user_btf_map(struct bpf_object *obj,
>                         }
>                         map->def.value_size = sz;
>                         map->btf_value_type_id = t->type;
> +               } else if (strcmp(name, "pinning") == 0) {
> +                       __u32 val;
> +                       int err;
> +
> +                       if (!get_map_field_int(map_name, obj->btf, def, m,
> +                                              &val))
> +                               return -EINVAL;
> +                       pr_debug("map '%s': found pinning = %u.\n",
> +                                map_name, val);
> +
> +                       if (val != LIBBPF_PIN_NONE &&
> +                           val != LIBBPF_PIN_BY_NAME) {
> +                               pr_warn("map '%s': invalid pinning value %u.\n",
> +                                       map_name, val);
> +                               return -EINVAL;
> +                       }
> +                       err = build_map_pin_path(map, pin_root_path);

uhm... only if (val == LIBBPF_PIN_BY_NAME)?.. maybe extend tests with
a mix if auto-pinned and never pinned map to catch issue like this?

> +                       if (err) {
> +                               pr_warn("map '%s': couldn't build pin path.\n",
> +                                       map_name);
> +                               return err;
> +                       }
>                 } else {
>                         if (strict) {
>                                 pr_warn("map '%s': unknown field '%s'.\n",
> @@ -1289,7 +1333,8 @@ static int bpf_object__init_user_btf_map(struct bpf_object *obj,
>         return 0;
>  }
>

[...]