Re: [PATCH] kbuild/btf: Remove broken module relinking exclusion

[Alexei Starovoitov]

On Tue, Apr 14, 2026 at 1:34 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Fri, Apr 10, 2026 at 03:13:29PM +0200, Petr Pavlu wrote:
> > Commit 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled
> > and pahole supports it") in 2020 introduced CONFIG_DEBUG_INFO_BTF_MODULES
> > to enable generation of split BTF for kernel modules. This change required
> > the %.ko Makefile rule to additionally depend on vmlinux, which is used as
> > a base for deduplication. The regular ld_ko_o command executed by the rule
> > was then modified to be skipped if only vmlinux changes. This was done by
> > introducing a new if_changed_except command and updating the original call
> > to '+$(call if_changed_except,ld_ko_o,vmlinux)'.
> >
> > Later, commit 214c0eea43b2 ("kbuild: add $(objtree)/ prefix to some
> > in-kernel build artifacts") in 2024 updated the rule's reference to vmlinux
> > from 'vmlinux' to '$(objtree)/vmlinux'. This accidentally broke the
> > previous logic to skip relinking modules if only vmlinux changes. The issue
> > is that '$(objtree)' is typically '.' and GNU Make normalizes the resulting
> > prerequisite './vmlinux' to just 'vmlinux', while the exclusion logic
> > retains the raw './vmlinux'. As a result, if_changed_except doesn't
> > correctly filter out vmlinux. Consequently, with
> > CONFIG_DEBUG_INFO_BTF_MODULES=y, modules are relinked even if only vmlinux
> > changes.
> >
> > It is possible to fix this Makefile issue. However, having the %.ko rule
> > update the resulting file in place without starting from the original
> > inputs is rather fragile. The logic is harder to debug if something breaks
> > during a subsequent .ko update because the old input is lost due to the
> > overwrite. Additionally, it requires that the BTF processing is idempotent.
> > For example, sorting id+flags BTF_SET8 pairs in .BTF_ids by resolve_btfids
> > currently doesn't have this property.
> >
> > One option is to split the %.ko target into two rules: the first for
> > partial linking and the second one for generating the BTF data. However,
> > this approach runs into an issue with requiring additional intermediate
> > files, which increases the size of the build directory. On my system, when
> > using a large distribution config with ~5500 modules, the size of the build
> > directory with debuginfo enabled is already ~25 GB, with .ko files
> > occupying ~8 GB. Duplicating these .ko files doesn't seem practical.
> >
> > Measuring the speed of the %.ko processing shows that the link step is
> > actually relatively fast. It takes about 20% of the overall rule time,
> > while the BTF processing accounts for 80%. Moreover, skipping the link part
> > becomes relevant only during local development. In such cases, developers
> > typically use configs that enable a limited number of modules, so having
> > the %.ko rule slightly slower doesn't significantly impact the total
> > rebuild time. This is supported by the fact that no one has complained
> > about this optimization being broken for the past two years.
> >
> > Therefore, remove the logic that prevents module relinking when only
> > vmlinux changes and simplify Makefile.modfinal.
> >
> > Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
>
> If the BPF folks want to take this since it deals with BTF:
>
> Acked-by: Nathan Chancellor <nathan@xxxxxxxxxx>
>
> Otherwise, either Nicolas can take this for 7.1 or I will pick it up for
> 7.2 when 7.1-rc1 is out.

Alan, Ihor,

As resident btf gen experts, Please take a look.