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

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Message-ID: <5c41d562589b497ca3c1047e0e18b3a1@AcuMS.aculab.com>
Date:   Wed, 21 Apr 2021 08:43:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arnd Bergmann' <arnd@...nel.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>
CC:     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

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
        .file   ""
        .globl  a
        .data
        .align 4
        .type   a, @object
        .size   a, 4
a:
        .long   8
        .ident  "GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609"
        .section        .note.GNU-stack,"",@progbits

Using '__alignof__(struct {long long x;})' does give the expected 4.

__alignof__() returns the preferred alignment, not the enforced
alignmnet - go figure.

	David

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