linux-kernel - Re: [PATCH] arm64: remove special treatment for the link order of head.o

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Message-ID: <CAADnVQLniq_NTN+dayioY76UvJ6Rt88wC31tboQx0UAvMn3Few@mail.gmail.com>
Date:   Fri, 24 Mar 2023 16:33:57 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Ard Biesheuvel <ardb@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Alan Maguire <alan.maguire@...cle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        Nicolas Schier <nicolas@...sle.eu>,
        linux-kernel@...r.kernel.org,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <bpf@...r.kernel.org>
Subject: Re: [PATCH] arm64: remove special treatment for the link order of head.o

On Fri, Mar 24, 2023 at 4:39 AM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> (cc BTF list and maintainer)
>
> On Thu, 23 Mar 2023 at 22:12, Aurelien Jarno <aurelien@...el32.net> wrote:
> >
> > Hi,
> >
> > On 2023-03-22 15:51, Ard Biesheuvel wrote:
> > > On Tue, 21 Mar 2023 at 23:26, Aurelien Jarno <aurelien@...el32.net> wrote:
> > > >
> > > > Hi,
> > > >
> > > > On 2022-10-13 08:35, Masahiro Yamada wrote:
> > > > > In the previous discussion (see the Link tag), Ard pointed out that
> > > > > arm/arm64/kernel/head.o does not need any special treatment - the only
> > > > > piece that must appear right at the start of the binary image is the
> > > > > image header which is emitted into .head.text.
> > > > >
> > > > > The linker script does the right thing to do. The build system does
> > > > > not need to manipulate the link order of head.o.
> > > > >
> > > > > Link: https://lore.kernel.org/lkml/CAMj1kXH77Ja8bSsq2Qj8Ck9iSZKw=1F8Uy-uAWGVDm4-CG=EuA@mail.gmail.com/
> > > > > Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> > > > > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > > > > ---
> > > > >
> > > > >  scripts/head-object-list.txt | 1 -
> > > > >  1 file changed, 1 deletion(-)
> > > > >
> > > > > diff --git a/scripts/head-object-list.txt b/scripts/head-object-list.txt
> > > > > index b16326a92c45..f226e45e3b7b 100644
> > > > > --- a/scripts/head-object-list.txt
> > > > > +++ b/scripts/head-object-list.txt
> > > > > @@ -15,7 +15,6 @@ arch/alpha/kernel/head.o
> > > > >  arch/arc/kernel/head.o
> > > > >  arch/arm/kernel/head-nommu.o
> > > > >  arch/arm/kernel/head.o
> > > > > -arch/arm64/kernel/head.o
> > > > >  arch/csky/kernel/head.o
> > > > >  arch/hexagon/kernel/head.o
> > > > >  arch/ia64/kernel/head.o
> > > >
> > > > This patch causes a significant increase of the arch/arm64/boot/Image
> > > > size. For instance the generic arm64 Debian kernel went from 31 to 39 MB
> > > > after this patch has been applied to the 6.1 stable tree.
> > > >
> > > > In turn this causes issues with some bootloaders, for instance U-Boot on
> > > > a Raspberry Pi limits the kernel size to 36 MB.
> > > >
> > >
> > > I cannot reproduce this with mainline
> > >
> > > With the patch
> > >
> > > $ size vmlinux
> > >    text    data     bss     dec     hex filename
> > > 24567309 14752630 621680 39941619 26175f3 vmlinux
> > >
> > > With the patch reverted
> > >
> > > $ size vmlinux
> > >    text    data     bss     dec     hex filename
> > > 24567309 14752694 621680 39941683 2617633 vmlinux
> >
> > I have tried with the current mainline, this is what I get, using GCC 12.2.0
> > and binutils 2.40:
> >
> >    text    data     bss     dec     hex filename
> > 32531655        8192996  621968 41346619        276e63b vmlinux.orig
> > 25170610        8192996  621968 33985574        2069426 vmlinux.revert
> >
> > > It would help to compare the resulting vmlinux ELF images from both
> > > builds to see where the extra space is being allocated
> >
> > At a first glance, it seems the extra space is allocated in the BTF
> > section. I have uploaded the resulting files as well as the config file
> > I used there:
> > https://temp.aurel32.net/linux-arm64-size-head.o.tar.gz
> >
>
> Indeed. So we go from
>
>   [15] .BTF              PROGBITS         ffff8000091d1ff4  011e1ff4
>        00000000005093d6  0000000000000000   A       0     0     1
>
> to
>
>   [15] .BTF              PROGBITS         ffff8000091d1ff4  011e1ff4
>        0000000000c0e5eb  0000000000000000   A       0     0     1
>
> i.e, from 5 MiB to 12+ MiB of BTF metadata.
>
> To me, it is not clear at all how one would be related to the other,
> so it will leave it to the Kbuild and BTF experts to chew on this one.

That's a huge increase.
It's not just that commit responsible, but the whole series ?
https://lore.kernel.org/lkml/20220906061313.1445810-1-masahiroy@kernel.org/
I'm guessing "Link vmlinux and modules in parallel" is related.
I'm not sure what "parallel link" means. Running 'ar' in parallel?
I cannot read makefile syntax, so no idea.

Jiri, Andrii, Alan, please take a look.