Subject: Re: [PATCH bpf-next v3 3/4] libbpf: Add auto-pinning of maps when loading BPF objects

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 29 Oct 2019 10:30:50 +0100

Andrii Nakryiko <[email-masked]> writes:

> On Sun, Oct 27, 2019 at 1:53 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> From: Toke Høiland-Jørgensen <[email-masked]>
>>
>> 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 <[email-masked]>
>> ---
>>  tools/lib/bpf/bpf_helpers.h |    6 ++
>>  tools/lib/bpf/libbpf.c      |  142 ++++++++++++++++++++++++++++++++++++++++++-
>>  tools/lib/bpf/libbpf.h      |   11 +++
>>  3 files changed, 154 insertions(+), 5 deletions(-)
>>
>
> [...]
>
>>
>> -static int bpf_object__init_maps(struct bpf_object *obj, bool relaxed_maps)
>> +static int bpf_object__build_map_pin_paths(struct bpf_object *obj,
>> +                                          const char *path)
>> +{
>> +       struct bpf_map *map;
>> +
>> +       if (!path)
>> +               path = "/sys/fs/bpf";
>> +
>> +       bpf_object__for_each_map(map, obj) {
>> +               char buf[PATH_MAX];
>> +               int err, len;
>> +
>> +               if (map->pinning != LIBBPF_PIN_BY_NAME)
>> +                       continue;
>
> still think it's better be done from map definition parsing code
> instead of a separate path, which will ignore most of maps anyways (of
> course by extracting this whole buffer creation logic into a
> function).

Hmm, okay, can do that. I think we should still store the actual value
of the 'pinning' attribute, though; and even have a getter for it. The
app may want to do something with that information instead of having to
infer it from map->pin_path. Certainly when we add other values of the
pinning attribute, but we may as well add the API to get the value
now...

>> +
>> +               len = snprintf(buf, PATH_MAX, "%s/%s", path, bpf_map__name(map));
>> +               if (len < 0)
>> +                       return -EINVAL;
>> +               else if (len >= PATH_MAX)
>
> [...]
>
>>         return 0;
>>  }
>>
>> +static bool map_is_reuse_compat(const struct bpf_map *map,
>> +                               int map_fd)
>
> nit: this should fit on single line?
>
>> +{
>> +       struct bpf_map_info map_info = {};
>> +       char msg[STRERR_BUFSIZE];
>> +       __u32 map_info_len;
>> +
>> +       map_info_len = sizeof(map_info);
>> +
>> +       if (bpf_obj_get_info_by_fd(map_fd, &map_info, &map_info_len)) {
>> +               pr_warn("failed to get map info for map FD %d: %s\n",
>> +                       map_fd, libbpf_strerror_r(errno, msg, sizeof(msg)));
>> +               return false;
>> +       }
>> +
>> +       return (map_info.type == map->def.type &&
>> +               map_info.key_size == map->def.key_size &&
>> +               map_info.value_size == map->def.value_size &&
>> +               map_info.max_entries == map->def.max_entries &&
>> +               map_info.map_flags == map->def.map_flags &&
>> +               map_info.btf_key_type_id == map->btf_key_type_id &&
>> +               map_info.btf_value_type_id == map->btf_value_type_id);
>
> If map was pinned by older version of the same app, key and value type
> id are probably gonna be different, even if the type definition itself
> it correct. We probably shouldn't check that?

Oh, I thought the type IDs would stay relatively stable. If not then I
agree that we shouldn't be checking them here. Will fix.

>> +}
>> +
>> +static int
>> +bpf_object__reuse_map(struct bpf_map *map)
>> +{
>> +       char *cp, errmsg[STRERR_BUFSIZE];
>> +       int err, pin_fd;
>> +
>> +       pin_fd = bpf_obj_get(map->pin_path);
>> +       if (pin_fd < 0) {
>> +               if (errno == ENOENT) {
>> +                       pr_debug("found no pinned map to reuse at '%s'\n",
>> +                                map->pin_path);
>> +                       return 0;
>> +               }
>> +
>> +               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> +               pr_warn("couldn't retrieve pinned map '%s': %s\n",
>> +                       map->pin_path, cp);
>> +               return -errno;
>
> store errno locally

*shrugs* okay, if you insist...

>> +       }
>> +
>> +       if (!map_is_reuse_compat(map, pin_fd)) {
>> +               pr_warn("couldn't reuse pinned map at '%s': "
>> +                       "parameter mismatch\n", map->pin_path);
>> +               close(pin_fd);
>> +               return -EINVAL;
>> +       }
>> +
>> +       err = bpf_map__reuse_fd(map, pin_fd);
>> +       if (err) {
>> +               close(pin_fd);
>> +               return err;
>> +       }
>> +       map->pinned = true;
>> +       pr_debug("reused pinned map at '%s'\n", map->pin_path);
>> +
>> +       return 0;
>> +}
>> +
>
> [...]
>
>> +enum libbpf_pin_type {
>> +       LIBBPF_PIN_NONE,
>> +       /* PIN_BY_NAME: pin maps by name (in /sys/fs/bpf by default) */
>> +       LIBBPF_PIN_BY_NAME,
>> +};
>> +
>>  LIBBPF_API int bpf_object__pin_maps(struct bpf_object *obj, const char *path);
>
> pin_maps should take into account opts->auto_pin_path, shouldn't it?
>
> Which is why I also think that auto_pin_path is bad name, because it's
> not only for auto-pinning, it's a pinning root path, so something like
> pin_root_path or just pin_root is better and less misleading name.

I view auto_pin_path as something that is used specifically for the
automatic pinning based on the 'pinning' attribute. Any other use of
pinning is for custom use and the user can pass a custom pin path to
those functions.