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

[David Laight]

From: Arnd Bergmann
> Sent: 20 April 2021 22:20
> 
> On Tue, Apr 20, 2021 at 11:14 PM Vineet Gupta
> <Vineet.Gupta1@xxxxxxxxxxxx> 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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