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From:   "Allan, Bruce W" <bruce.w.allan@intel.com>
To:     Song Liu <songliubraving@fb.com>,
        Andrii Nakryiko <andrii@kernel.org>
CC:     bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
        "Starovoitov, Alexei" <ast@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Kernel Team <Kernel-team@fb.com>,
        open list <linux-kernel@vger.kernel.org>,
        "rafael@kernel.org" <rafael@kernel.org>,
        "jeyu@kernel.org" <jeyu@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>
Subject: RE: [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if BTF
 is enabled and pahole supports it
Thread-Topic: [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if BTF
 is enabled and pahole supports it
Thread-Index: AQHWt8cxfXnADugwHkCEjLA2hSU/rKnLJ4Tw
Date:   Mon, 16 Nov 2020 19:55:15 +0000
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> -----Original Message-----
> From: Song Liu <songliubraving@fb.com>
> Sent: Tuesday, November 10, 2020 5:05 PM
> To: Andrii Nakryiko <andrii@kernel.org>
> Cc: bpf <bpf@vger.kernel.org>; Networking <netdev@vger.kernel.org>;
> Starovoitov, Alexei <ast@fb.com>; Daniel Borkmann <daniel@iogearbox.net>;
> Kernel Team <Kernel-team@fb.com>; open list <linux-
> kernel@vger.kernel.org>; rafael@kernel.org; jeyu@kernel.org; Arnaldo
> Carvalho de Melo <acme@redhat.com>; Greg Kroah-Hartman
> <gregkh@linuxfoundation.org>; Masahiro Yamada
> <yamada.masahiro@socionext.com>
> Subject: Re: [PATCH v4 bpf-next 3/5] kbuild: build kernel module BTFs if =
BTF is
> enabled and pahole supports it
>=20
>=20
>=20
> > On Nov 9, 2020, at 5:19 PM, Andrii Nakryiko <andrii@kernel.org> wrote:
>=20
> [...]
>=20
> > SPLIT BTF
> > =3D=3D=3D=3D=3D=3D=3D=3D=3D
> >
> > $ for f in $(find . -name '*.ko'); do size -A -d $f | grep BTF | awk '{=
print $2}';
> done | awk '{ s +=3D $1 } END { print s }'
> > 5194047
> >
> > $ for f in $(find . -name '*.ko'); do printf "%s %d\n" $f $(size -A -d =
$f | grep
> BTF | awk '{print $2}'); done | sort -nr -k2 | head -n10
> > ./drivers/gpu/drm/i915/i915.ko 293206
> > ./drivers/gpu/drm/radeon/radeon.ko 282103
> > ./fs/xfs/xfs.ko 222150
> > ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko 198503
> > ./drivers/infiniband/hw/mlx5/mlx5_ib.ko 198356
> > ./drivers/net/ethernet/broadcom/bnx2x/bnx2x.ko 113444
> > ./fs/cifs/cifs.ko 109379
> > ./arch/x86/kvm/kvm.ko 100225
> > ./drivers/gpu/drm/drm.ko 94827
> > ./drivers/infiniband/core/ib_core.ko 91188
> >
> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>=20
> Acked-by: Song Liu <songliubraving@fb.com>

This change, commit 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF =
is enabled and pahole
supports it") currently in net-next, linux-next, etc. breaks the use-case o=
f compiling only a specific
kernel module (both in-tree and out-of-tree, e.g. 'make M=3Ddrivers/net/eth=
ernet/intel/ice') after
first doing a 'make modules_prepare'.  Previously, that use-case would resu=
lt in a warning noting
"Symbol info of vmlinux is missing. Unresolved symbol check will be entirel=
y skipped" but now it
errors out after noting "No rule to make target 'vmlinux', needed by '<...>=
.ko'.  Stop."

Is that intentional?

Thanks,
Bruce.