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From: David Laight <David.Laight@ACULAB.COM>
To: 'Arnd Bergmann' <arnd@kernel.org>,
	Vineet Gupta <Vineet.Gupta1@synopsys.com>
Cc: "grygorii.strashko@ti.com" <grygorii.strashko@ti.com>,
	"linux-snps-arc@lists.infradead.org"
	<linux-snps-arc@lists.infradead.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"ilias.apalodimas@linaro.org" <ilias.apalodimas@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	"linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"mgorman@suse.de" <mgorman@suse.de>,
	"brouer@redhat.com" <brouer@redhat.com>,
	"mcroce@linux.microsoft.com" <mcroce@linux.microsoft.com>,
	"mhocko@kernel.org" <mhocko@kernel.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	"hch@lst.de" <hch@lst.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
Date: Wed, 21 Apr 2021 08:43:01 +0000
Message-ID: <5c41d562589b497ca3c1047e0e18b3a1@AcuMS.aculab.com> (raw)
In-Reply-To: <CAK8P3a3rzz1gfNLoGC8aZJiAC-tgZYD6P8pQsoEfgCAmQK=FAw@mail.gmail.com>

From: Arnd Bergmann
> Sent: 20 April 2021 22:20
> 
> On Tue, Apr 20, 2021 at 11:14 PM Vineet Gupta
> <Vineet.Gupta1@synopsys.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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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  parent reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-16 23:07 [PATCH 0/2] Change struct page layout for page_pool Matthew Wilcox (Oracle)
2021-04-16 23:07 ` [PATCH 1/2] mm: Fix struct page layout on 32-bit systems Matthew Wilcox (Oracle)
2021-04-17  1:08   ` kernel test robot
2021-04-17  2:45   ` Matthew Wilcox
2021-04-17 18:32     ` Ilias Apalodimas
2021-04-17 20:22       ` Matthew Wilcox
2021-04-18 11:21         ` Ilias Apalodimas
2021-04-17 21:18     ` David Laight
2021-04-17 22:45       ` Matthew Wilcox
2021-04-20  2:48     ` Vineet Gupta
2021-04-20  3:10       ` Matthew Wilcox
2021-04-20  7:07         ` Arnd Bergmann
2021-04-20 21:14           ` Vineet Gupta
2021-04-20 21:20             ` Arnd Bergmann
2021-04-21  5:50               ` hch
2021-04-21  8:43               ` David Laight [this message]
2021-04-21  8:58                 ` Arnd Bergmann
2021-04-20  7:39     ` Geert Uytterhoeven
2021-04-20  8:39       ` David Laight
2021-04-20 11:32       ` Matthew Wilcox
2021-04-17  7:34   ` Jesper Dangaard Brouer
2021-04-20  7:21   ` Rasmus Villemoes
2021-04-16 23:07 ` [PATCH 2/2] mm: Indicate pfmemalloc pages in compound_head Matthew Wilcox (Oracle)
2021-04-17 21:13   ` David Laight
2021-04-17 21:28     ` Matthew Wilcox
2021-04-20  4:03 ` [PATCH 0/2] Change struct page layout for page_pool Michael Ellerman

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