> On Oct 28, 2020, at 5:58 PM, Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
>
> Add support for deduplication split BTFs. When deduplicating split BTF, base
> BTF is considered to be immutable and can't be modified or adjusted. 99% of
> BTF deduplication logic is left intact (module some type numbering adjustments).
> There are only two differences.
>
> First, each type in base BTF gets hashed (expect VAR and DATASEC, of course,
> those are always considered to be self-canonical instances) and added into
> a table of canonical table candidates. Hashing is a shallow, fast operation,
> so mostly eliminates the overhead of having entire base BTF to be a part of
> BTF dedup.
>
> Second difference is very critical and subtle. While deduplicating split BTF
> types, it is possible to discover that one of immutable base BTF BTF_KIND_FWD
> types can and should be resolved to a full STRUCT/UNION type from the split
> BTF part. This is, obviously, can't happen because we can't modify the base
> BTF types anymore. So because of that, any type in split BTF that directly or
> indirectly references that newly-to-be-resolved FWD type can't be considered
> to be equivalent to the corresponding canonical types in base BTF, because
> that would result in a loss of type resolution information. So in such case,
> split BTF types will be deduplicated separately and will cause some
> duplication of type information, which is unavoidable.
>
> With those two changes, the rest of the algorithm manages to deduplicate split
> BTF correctly, pointing all the duplicates to their canonical counter-parts in
> base BTF, but also is deduplicating whatever unique types are present in split
> BTF on their own.
>
> Also, theoretically, split BTF after deduplication could end up with either
> empty type section or empty string section. This is handled by libbpf
> correctly in one of previous patches in the series.
>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Song Liu <songliubraving@xxxxxx>
With some nits:
> ---
[...]
>
> /* remap string offsets */
> err = btf_for_each_str_off(d, strs_dedup_remap_str_off, d);
> @@ -3553,6 +3582,63 @@ static bool btf_compat_fnproto(struct btf_type *t1, struct btf_type *t2)
> return true;
> }
>
An overview comment about bpf_deup_prep() will be great.
> +static int btf_dedup_prep(struct btf_dedup *d)
> +{
> + struct btf_type *t;
> + int type_id;
> + long h;
> +
> + if (!d->btf->base_btf)
> + return 0;
> +
> + for (type_id = 1; type_id < d->btf->start_id; type_id++)
> + {
Move "{" to previous line?
> + t = btf_type_by_id(d->btf, type_id);
> +
> + /* all base BTF types are self-canonical by definition */
> + d->map[type_id] = type_id;
> +
> + switch (btf_kind(t)) {
> + case BTF_KIND_VAR:
> + case BTF_KIND_DATASEC:
> + /* VAR and DATASEC are never hash/deduplicated */
> + continue;
[...]
> /* we are going to reuse hypot_map to store compaction remapping */
> d->hypot_map[0] = 0;
> - for (i = 1; i <= d->btf->nr_types; i++)
> - d->hypot_map[i] = BTF_UNPROCESSED_ID;
> + /* base BTF types are not renumbered */
> + for (id = 1; id < d->btf->start_id; id++)
> + d->hypot_map[id] = id;
> + for (i = 0, id = d->btf->start_id; i < d->btf->nr_types; i++, id++)
> + d->hypot_map[id] = BTF_UNPROCESSED_ID;
We don't really need i in the loop, shall we just do
for (id = d->btf->start_id; id < d->btf->start_id + d->btf->nr_types; id++)
?
>
> p = d->btf->types_data;
>
> - for (i = 1; i <= d->btf->nr_types; i++) {
> - if (d->map[i] != i)
> + for (i = 0, id = d->btf->start_id; i < d->btf->nr_types; i++, id++) {
ditto
> + if (d->map[id] != id)
> continue;
>
[...]
