linux-kernel - Re: Build regressions/improvements in v4.17-rc1

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Date:   Wed, 08 Aug 2018 23:16:16 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "npiggin\@gmail.com" <npiggin@...il.com>
Subject: Re: Build regressions/improvements in v4.17-rc1

Michael Ellerman <mpe@...erman.id.au> writes:
> Andrew Morton <akpm@...ux-foundation.org> writes:
>> On Mon, 6 Aug 2018 12:39:21 +0200 Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>
>>> CC Dan, Michael, AKPM, powerpc
>>> 
>>> On Mon, Apr 16, 2018 at 3:10 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>>> > Below is the list of build error/warning regressions/improvements in
>>> > v4.17-rc1[1] compared to v4.16[2].
>>> 
>>> I'd like to point your attention to:
>>> 
>>> >   + warning: vmlinux.o(.text+0x376518): Section mismatch in reference from the function .devm_memremap_pages() to the function .meminit.text:.arch_add_memory():  => N/A
>>> >   + warning: vmlinux.o(.text+0x376d64): Section mismatch in reference from the function .devm_memremap_pages_release() to the function .meminit.text:.arch_remove_memory():  => N/A
>>
>> hm.  Dan isn't around at present so we're on our own with this one.
>>
>> x86 doesn't put arch_add_memory and arch_remove_memory into __meminit. 
>> x86 does
>>
>> #ifdef CONFIG_MEMORY_HOTPLUG
>> int arch_add_memory(int nid, u64 start, u64 size, struct vmem_altmap *altmap,
>> 		bool want_memblock)
>> {
>> 	...
>>
>>
>> So I guess powerpc should do that as well?
>
> But we only recently added it to fix a section mismatch warning:
>
>   WARNING: vmlinux.o(.text+0x6da88): Section mismatch in reference from the function .arch_add_memory() to the function .meminit.text:.create_section_mapping()
>   The function .arch_add_memory() references
>   the function __meminit .create_section_mapping().
>   This is often because .arch_add_memory lacks a __meminit 
>   annotation or the annotation of .create_section_mapping is wrong.
>
>
> I think the problem is that the section mismatch logic isn't able to
> cope with __meminit's changing semantics.
>
> When CONFIG_MEMORY_HOTPLUG=y references from .text to .meminit.text
> should be allowed, because they're just folded in together in the linker
> script.
>
> When CONFIG_MEMORY_HOTPLUG=n references from .text to .meminit.text
> should NOT be allowed, because .meminit.text becomes .init.text and will
> be freed.
>
> I don't see anything in the section mismatch logic to cope with that
> difference.
>
> It looks like __meminit is saving us about 1K on powerpc, so I'm
> strongly inclined to just remove it entirely from arch/powerpc.

Gah that doesn't work.

Our arch routines call things in mm/ that are __meminit, so we have to
mark our code __meminit too, otherwise we get a section mismatch
warning.

cheers