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Miller" , Daniel Borkmann , Andrii Nakryiko , Networking , bpf , Kernel Team Subject: Re: [PATCH v2 bpf-next 09/16] libbpf: Support for fd_idx Message-ID: <20210428013242.2iqeygfpmoyzwvxh@ast-mbp.dhcp.thefacebook.com> References: <20210423002646.35043-1-alexei.starovoitov@gmail.com> <20210423002646.35043-10-alexei.starovoitov@gmail.com> <20210427025309.yma2vy4m4qbk5srv@ast-mbp.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, Apr 27, 2021 at 09:36:54AM -0700, Andrii Nakryiko wrote: > On Mon, Apr 26, 2021 at 7:53 PM Alexei Starovoitov > wrote: > > > > On Mon, Apr 26, 2021 at 10:14:45AM -0700, Andrii Nakryiko wrote: > > > On Thu, Apr 22, 2021 at 5:27 PM Alexei Starovoitov > > > wrote: > > > > > > > > From: Alexei Starovoitov > > > > > > > > Add support for FD_IDX make libbpf prefer that approach to loading programs. > > > > > > > > Signed-off-by: Alexei Starovoitov > > > > --- > > > > tools/lib/bpf/bpf.c | 1 + > > > > tools/lib/bpf/libbpf.c | 70 +++++++++++++++++++++++++++++---- > > > > tools/lib/bpf/libbpf_internal.h | 1 + > > > > 3 files changed, 65 insertions(+), 7 deletions(-) > > > > > > > > > [...] > > > > > for (i = 0; i < obj->nr_programs; i++) { > > > > prog = &obj->programs[i]; > > > > if (prog_is_subprog(obj, prog)) > > > > @@ -7256,10 +7308,14 @@ bpf_object__load_progs(struct bpf_object *obj, int log_level) > > > > continue; > > > > } > > > > prog->log_level |= log_level; > > > > + prog->fd_array = fd_array; > > > > > > you are not freeing this memory on success, as far as I can see. > > > > hmm. there is free on success below. > > right, my bad, I somehow understood as if it was only for error case > > > > > > And > > > given multiple programs are sharing fd_array, it's a bit problematic > > > for prog to have fd_array. This is per-object properly, so let's add > > > it at bpf_object level and clean it up on bpf_object__close()? And by > > > assigning to obj->fd_array at malloc() site, you won't need to do all > > > the error-handling free()s below. > > > > hmm. that sounds worse. > > why add another 8 byte to bpf_object that won't be used > > until this last step of bpf_object__load_progs. > > And only for the duration of this loading. > > It's cheaper to have this alloc here with two free()s below. > > So if you care about extra 8 bytes, then it's even more efficient to > have just one obj->fd_array rather than N prog->fd_array, no? I think it's layer breaking when bpf_program__load()->load_program() has to reach out to prog->obj to do its work. The layers are already a mess due to: &prog->obj->maps[prog->obj->rodata_map_idx] I wanted to avoid making it uglier. > And it's > also not very clean that prog->fd_array will have a dangling pointer > to deallocated memory after bpf_object__load_progs(). prog->reloc_desc is free and zeroed after __relocate. prog->insns are freed and _not_ zereod after __load_progs. so prog->fd_array won't be the first such pointer. I can add zeroing, of course. > > But that brings the entire question of why use fd_array at all here? > Commit description doesn't explain why libbpf has to use fd_array and > why it should be preferred. What are the advantages justifying added > complexity and extra memory allocation/clean up? It also reduces test > coverage of the "old ways" that offer the same capabilities. I think > this should be part of the commit description, if we agree that > fd_array has to be used outside of the auto-generated loader program. I can add a knob to it to use it during loader gen for the loader gen and for the runner of the loader prog. I think it will add more complexity. The bpf CI runs on older kernels, so the test coverage of "old ways" is not reduced regardless. >From the kernel pov BPF_PSEUDO_MAP_FD vs BPF_PSEUDO_MAP_IDX there is no advantage. >From the libbpf side patch 9 looked trivial enough _not_ do it conditionally, but whatever. I don't mind more 'if'-s.