linux-kernel - Re: [RFC PATCH v4 2/7] libbpf: Add BTF permutation support for type reordering

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Message-ID: <CAErzpmv+wmfh_hjSiR5WhSyXVmX-iFAw3cYo5qeMphsWayyZWw@mail.gmail.com>
Date: Wed, 5 Nov 2025 19:31:45 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: ast@...nel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	Andrii Nakryiko <andrii.nakryiko@...il.com>, Alan Maguire <alan.maguire@...cle.com>, 
	Song Liu <song@...nel.org>, pengdonglin <pengdonglin@...omi.com>
Subject: Re: [RFC PATCH v4 2/7] libbpf: Add BTF permutation support for type reordering

On Wed, Nov 5, 2025 at 7:45 AM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Tue, 2025-11-04 at 21:40 +0800, Donglin Peng wrote:
> > From: pengdonglin <pengdonglin@...omi.com>
> >
> > Introduce btf__permute() API to allow in-place rearrangement of BTF types.
> > This function reorganizes BTF type order according to a provided array of
> > type IDs, updating all type references to maintain consistency.
> >
> > The permutation process involves:
> > 1. Shuffling types into new order based on the provided ID mapping
> > 2. Remapping all type ID references to point to new locations
> > 3. Handling BTF extension data if provided via options
> >
> > This is particularly useful for optimizing type locality after BTF
> > deduplication or for meeting specific layout requirements in specialized
> > use cases.
> >
> > Cc: Eduard Zingerman <eddyz87@...il.com>
> > Cc: Alexei Starovoitov <ast@...nel.org>
> > Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>
> > Cc: Alan Maguire <alan.maguire@...cle.com>
> > Cc: Song Liu <song@...nel.org>
> > Signed-off-by: pengdonglin <pengdonglin@...omi.com>
> > Signed-off-by: Donglin Peng <dolinux.peng@...il.com>
>
> Acked-by: Eduard Zingerman <eddyz87@...il.com>
>
> [...]
>
> > --- a/tools/lib/bpf/btf.h
> > +++ b/tools/lib/bpf/btf.h
> > @@ -273,6 +273,40 @@ LIBBPF_API int btf__dedup(struct btf *btf, const struct btf_dedup_opts *opts);
> >   */
> >  LIBBPF_API int btf__relocate(struct btf *btf, const struct btf *base_btf);
> >
> > +struct btf_permute_opts {
> > +     size_t sz;
> > +     /* optional .BTF.ext info along the main BTF info */
> > +     struct btf_ext *btf_ext;
> > +     size_t :0;
> > +};
> > +#define btf_permute_opts__last_field btf_ext
> > +
> > +/**
> > + * @brief **btf__permute()** rearranges BTF types in-place according to specified mapping
> > + * @param btf BTF object to permute
> > + * @param ids Array defining new type order. Must contain exactly btf->nr_types elements,
> > + *        each being a valid type ID in range [btf->start_id, btf->start_id + btf->nr_types - 1]
> > + * @param opts Optional parameters, including BTF extension data for reference updates
> > + * @return 0 on success, negative error code on failure
> > + *
> > + * **btf__permute()** performs an in-place permutation of BTF types, rearranging them
> > + * according to the order specified in @p ids array. After reordering, all type references
> > + * within the BTF data and optional BTF extension are updated to maintain consistency.
> > + *
> > + * The permutation process consists of two phases:
> > + * 1. Type shuffling: Physical reordering of type data in memory
> > + * 2. Reference remapping: Updating all type ID references to new locations
>
> Nit: Please drop this paragraph: it is an implementation detail, not
>      user-facing behavior, and it is obvious from the function code.

Thanks, I will fix it in the next version.

>
> > + *
> > + * This is particularly useful for optimizing type locality after BTF deduplication
> > + * or for meeting specific layout requirements in specialized use cases.
>
> Nit: Please drop this paragraph as well.

Thanks, I will fix it in the next version.

>
> > + *
> > + * On error, negative error code is returned and errno is set appropriately.
> > + * Common error codes include:
> > + *   - -EINVAL: Invalid parameters or invalid ID mapping (e.g., duplicate IDs, out-of-range IDs)
> > + *   - -ENOMEM: Memory allocation failure during permutation process
> > + */
> > +LIBBPF_API int btf__permute(struct btf *btf, __u32 *ids, const struct btf_permute_opts *opts);
> > +
> >  struct btf_dump;
> >
> >  struct btf_dump_opts {
>
> [...]