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

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Date:   Wed, 21 Apr 2021 10:58:18 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Matthew Wilcox <willy@...radead.org>,
        "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 Wed, Apr 21, 2021 at 10:43 AM David Laight <David.Laight@...lab.com> wrote:
> From: Arnd Bergmann Sent: 20 April 2021 22:20
> > On Tue, Apr 20, 2021 at 11:14 PM Vineet Gupta <Vineet.Gupta1@...opsys.com> wrote:
> > > On 4/20/21 12:07 AM, Arnd Bergmann wrote:
> >
> > > >
> > > > 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?
> > >
> > > $ echo 'int a = __alignof__(long long);' | arc64-linux-gnu-gcc -xc -
> > > -Wall -S -o - | grep -A1 a: | tail -n 1 | cut -f 3
> > > 8
> >
> > Ok, good.
>
> That test doesn't prove anything.
> Try running on x86:
> $ echo 'int a = __alignof__(long long);' | gcc -xc - -Wall -S -o - -m32
> a:
>         .long   8

Right, I had wondered about that one after I sent the email.

> Using '__alignof__(struct {long long x;})' does give the expected 4.
>
> __alignof__() returns the preferred alignment, not the enforced
> alignmnet - go figure.

I checked the others as well now, and i386 is the only one that
changed here: m68k still has '2', while arc/csky/h8300/microblaze/
nios2/or1k/sh/i386 all have '4' and the rest have '8'.

     Arnd