linux-kernel - Re: [ANNOUNCE] "Fast Kernel Headers" Tree -v2

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Message-ID: <Yd/idffvv8QIQcEU@gmail.com>
Date:   Thu, 13 Jan 2022 09:27:33 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [ANNOUNCE] "Fast Kernel Headers" Tree -v2


* Arnd Bergmann <arnd@...db.de> wrote:

> On Mon, Jan 10, 2022 at 11:03 PM Arnd Bergmann <arnd@...db.de> wrote:
> > On Sat, Jan 8, 2022 at 5:26 PM Ingo Molnar <mingo@...nel.org> wrote:
> > I've started building randconfig kernels for arm64 and x86, and fixing
> > up things that come up, a few things I have noticed out so far:
> 
> I have run into a couple more specific issues:
> 
> * net/smc/smc_ib.c:824:26: error: implicit declaration of function
> 'cache_line_size' [-Werror=implicit-function-declaration]
> cache_line_size is generally provided by linux/cache.h, which includes
> asm/cache.h.
> This works on arm64, but not on x86, where asm/cache.h would have to include
> asm/cpufeature.h, and but it would be good to avoid that because of the implicit
> linux/percpu.h and linux/bitops.h inclusions. Also, if I add the
> include, I get this
> build failure instead: include/linux/smp_types.h:88:33: error:
> requested alignment '20'
> is not a positive power of 2

Note that this particular one should be fixed in the WIP branch, which is 
at:

   git://git.kernel.org/pub/scm/linux/kernel/git/mingo/tip.git sched/headers

> * arm64 has a couple of issues around asm/memory.h, linux/mm_types.h and 
> asm/page.h that can cause loops. I think my latest version has it figured 
> out, but there is probably room for optimization.

Yeah, this is like the 5th attempt at finding a robust solution. :-/

> * There is no general way to get the get_order() definition, other than 
> including asm/page.h from .c files. On arm64, this shows up in a couple 
> of files after the cleanup. Only xtensa and ia64 define their own version 
> of get_order(), and I think we should just remove those and move the 
> generic version to linux/getorder.h, where any file using it can pick it 
> up. For randconfig builds, I had to add asm/page.h to 
> net/xdp/xsk_queue.c, mm/memtest.c and 
> drivers/target/iscsi/iscsi_target_nego.c, after I removed the indirect 
> include from arch/arm64/include/asm/mmu.h in the previous step.

Would including <linux/mm_page_address.h> be sufficient? That already has 
an <asm/page.h> inclusion and is vaguely related.

I tried to avoid as many low level headers as possible from the main types 
headers - and the get_order() functionality also brings in bitops 
definitions, which I'm still hoping to be able to reduce from its current 
~95% utilization in a distro kernel ...

We could add <linux/page_api.h> as well, as a standardized header. We 
already have page_types.h and et_order() is a page types API.

Thanks,

	Ingo