[External] Re: [PATCH bpf-next v2 7/9] libbpf: don't free btf if program of multi-link tracing existing

[梦龙董]

On Tue, Mar 12, 2024 at 9:55 AM Alexei Starovoitov
<alexei.starovoitov at gmail.com> wrote:
>
> On Mon, Mar 11, 2024 at 2:35 AM Menglong Dong
> <dongmenglong.8 at bytedance.com> wrote:
> >
> > By default, the kernel btf that we load during loading program will be
> > freed after the programs are loaded in bpf_object_load(). However, we
> > still need to use these btf for tracing of multi-link during attaching.
> > Therefore, we don't free the btfs until the bpf object is closed if any
> > bpf programs of the type multi-link tracing exist.
> >
> > Meanwhile, introduce the new api bpf_object__free_btf() to manually free
> > the btfs after attaching.
> >
> > Signed-off-by: Menglong Dong <dongmenglong.8 at bytedance.com>
> > ---
> >  tools/lib/bpf/libbpf.c   | 47 ++++++++++++++++++++++++++++++----------
> >  tools/lib/bpf/libbpf.h   |  2 ++
> >  tools/lib/bpf/libbpf.map |  1 +
> >  3 files changed, 38 insertions(+), 12 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 567ad367e7aa..fd5428494a7e 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -8267,6 +8267,39 @@ static int bpf_object_prepare_struct_ops(struct bpf_object *obj)
> >         return 0;
> >  }
> >
> > +void bpf_object__free_btfs(struct bpf_object *obj)
> > +{
> > +       int i;
> > +
> > +       /* clean up module BTFs */
> > +       for (i = 0; i < obj->btf_module_cnt; i++) {
> > +               close(obj->btf_modules[i].fd);
> > +               btf__free(obj->btf_modules[i].btf);
> > +               free(obj->btf_modules[i].name);
> > +       }
> > +       free(obj->btf_modules);
> > +       obj->btf_modules = NULL;
> > +       obj->btf_module_cnt = 0;
> > +
> > +       /* clean up vmlinux BTF */
> > +       btf__free(obj->btf_vmlinux);
> > +       obj->btf_vmlinux = NULL;
> > +}
> > +
> > +static void bpf_object_early_free_btf(struct bpf_object *obj)
> > +{
> > +       struct bpf_program *prog;
> > +
> > +       bpf_object__for_each_program(prog, obj) {
> > +               if (prog->expected_attach_type == BPF_TRACE_FENTRY_MULTI ||
> > +                   prog->expected_attach_type == BPF_TRACE_FEXIT_MULTI ||
> > +                   prog->expected_attach_type == BPF_MODIFY_RETURN_MULTI)
> > +                       return;
> > +       }
> > +
> > +       bpf_object__free_btfs(obj);
> > +}
> > +
> >  static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const char *target_btf_path)
> >  {
> >         int err, i;
> > @@ -8307,18 +8340,7 @@ static int bpf_object_load(struct bpf_object *obj, int extra_log_level, const ch
> >         /* clean up fd_array */
> >         zfree(&obj->fd_array);
> >
> > -       /* clean up module BTFs */
> > -       for (i = 0; i < obj->btf_module_cnt; i++) {
> > -               close(obj->btf_modules[i].fd);
> > -               btf__free(obj->btf_modules[i].btf);
> > -               free(obj->btf_modules[i].name);
> > -       }
> > -       free(obj->btf_modules);
> > -
> > -       /* clean up vmlinux BTF */
> > -       btf__free(obj->btf_vmlinux);
> > -       obj->btf_vmlinux = NULL;
> > -
> > +       bpf_object_early_free_btf(obj);
> >         obj->loaded = true; /* doesn't matter if successfully or not */
> >
> >         if (err)
> > @@ -8791,6 +8813,7 @@ void bpf_object__close(struct bpf_object *obj)
> >         usdt_manager_free(obj->usdt_man);
> >         obj->usdt_man = NULL;
> >
> > +       bpf_object__free_btfs(obj);
> >         bpf_gen__free(obj->gen_loader);
> >         bpf_object__elf_finish(obj);
> >         bpf_object_unload(obj);
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 5723cbbfcc41..c41a909ea4c1 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -299,6 +299,8 @@ LIBBPF_API struct bpf_program *
> >  bpf_object__find_program_by_name(const struct bpf_object *obj,
> >                                  const char *name);
> >
> > +LIBBPF_API void bpf_object__free_btfs(struct bpf_object *obj);
> > +
>
> It shouldn't be exported.
> libbpf should clean it up when bpf_object is freed.

Yes, libbpf will clean up the btfs when bpf_object is freed in
this commit. And I'm trying to offer a way to early free the btfs
by the users manual to reduce the memory usage. Or, the
btfs that we opened will keep existing until we close the
bpf_object.

This is optional, I can remove it if you prefer.

Thanks!
Menglong Dong