linux-kernel - Re: [PATCH 1/7] mm: allocate mm_cpumask dynamically based on nr_cpu

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Message-ID: <20180804222808.GA8011@roeck-us.net>
Date:   Sat, 4 Aug 2018 15:28:08 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, luto@...nel.org,
        efault@....de, kernel-team@...com, mingo@...nel.org,
        dave.hansen@...el.com, Guan Xuetao <gxt@....edu.cn>
Subject: Re: [PATCH 1/7] mm: allocate mm_cpumask dynamically based on
 nr_cpu_ids

On Mon, Jul 16, 2018 at 03:03:31PM -0400, Rik van Riel wrote:
> The mm_struct always contains a cpumask bitmap, regardless of
> CONFIG_CPUMASK_OFFSTACK. That means the first step can be to
> simplify things, and simply have one bitmask at the end of the
> mm_struct for the mm_cpumask.
> 
> This does necessitate moving everything else in mm_struct into
> an anonymous sub-structure, which can be randomized when struct
> randomization is enabled.
> 
> The second step is to determine the correct size for the
> mm_struct slab object from the size of the mm_struct
> (excluding the cpu bitmap) and the size the cpumask.
> 
> For init_mm we can simply allocate the maximum size this
> kernel is compiled for, since we only have one init_mm
> in the system, anyway.
> 
> Pointer magic by Mike Galbraith, to evade -Wstringop-overflow
> getting confused by the dynamically sized array.
> 
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Mike Galbraith <efault@....de>
> Signed-off-by: Rik van Riel <riel@...riel.com>
> Acked-by: Dave Hansen <dave.hansen@...el.com>
> Tested-by: Song Liu <songliubraving@...com>

Hi,

this patch causes unicore32 build failures.

In file included from include/linux/mm.h:17,
                 from arch/unicore32/kernel/asm-offsets.c:17:
include/linux/mm_types.h:497: error:
	flexible array member in otherwise empty struct

Build reference: next-20180803
gcc version: unicore32-linux-gcc (UC4_1.0.5_20100917) 4.4.2

I understand this is an old compiler, but it is the only available
version as far as I know.

Guenter