netdev - Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

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Date:   Tue, 20 Apr 2021 09:07:55 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        "grygorii.strashko@...com" <grygorii.strashko@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "mcroce@...ux.microsoft.com" <mcroce@...ux.microsoft.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "hch@....de" <hch@....de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

On Tue, Apr 20, 2021 at 5:10 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Apr 20, 2021 at 02:48:17AM +0000, Vineet Gupta wrote:
> > > 32-bit architectures which expect 8-byte alignment for 8-byte integers
> > > and need 64-bit DMA addresses (arc, arm, mips, ppc) had their struct
> > > page inadvertently expanded in 2019.
> >
> > FWIW, ARC doesn't require 8 byte alignment for 8 byte integers. This is
> > only needed for 8-byte atomics due to the requirements of LLOCKD/SCOND
> > instructions.
>
> Ah, like x86?  OK, great, I'll drop your arch from the list of
> affected.  Thanks!

I mistakenly assumed that i386 and m68k were the only supported
architectures with 32-bit alignment on u64. I checked it now and found

$ for i in /home/arnd/cross/x86_64/gcc-10.1.0-nolibc/*/bin/*-gcc ; do
echo `echo 'int a = __alignof__(long long);' | $i -xc - -Wall -S -o- |
grep -A1 a: | tail -n 1 | cut -f 3 -d\   `
${i#/home/arnd/cross/x86_64/gcc-10.1.0-nolibc/*/bin/} ; done
8 aarch64-linux-gcc
8 alpha-linux-gcc
4 arc-linux-gcc
8 arm-linux-gnueabi-gcc
8 c6x-elf-gcc
4 csky-linux-gcc
4 h8300-linux-gcc
8 hppa-linux-gcc
8 hppa64-linux-gcc
8 i386-linux-gcc
8 ia64-linux-gcc
2 m68k-linux-gcc
4 microblaze-linux-gcc
8 mips-linux-gcc
8 mips64-linux-gcc
8 nds32le-linux-gcc
4 nios2-linux-gcc
4 or1k-linux-gcc
8 powerpc-linux-gcc
8 powerpc64-linux-gcc
8 riscv32-linux-gcc
8 riscv64-linux-gcc
8 s390-linux-gcc
4 sh2-linux-gcc
4 sh4-linux-gcc
8 sparc-linux-gcc
8 sparc64-linux-gcc
8 x86_64-linux-gcc
8 xtensa-linux-gcc

which means that half the 32-bit architectures do this. This may
cause more problems when arc and/or microblaze want to support
64-bit kernels and compat mode in the future on their latest hardware,
as that means duplicating the x86 specific hacks we have for compat.

What is alignof(u64) on 64-bit arc?

      Arnd