Re: [PATCH 1/7] mm: allocate mm_cpumask dynamically based on nr_cpu_ids

From: Rik van Riel <riel@surriel.com>
To: Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org
Cc: x86@kernel.org, luto@kernel.org, dave.hansen@linux.intel.com,
	mingo@kernel.org, kernel-team@fb.com, tglx@linutronix.de,
	songliubraving@fb.com, hpa@zytor.com
Subject: Re: [PATCH 1/7] mm: allocate mm_cpumask dynamically based on nr_cpu_ids
Date: Sat, 07 Jul 2018 17:25:09 -0400	[thread overview]
Message-ID: <1530998709.5350.32.camel@surriel.com> (raw)
In-Reply-To: <1530951813.22672.10.camel@gmx.de>

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   I. On Sat, 2018-07-07 at 10:23 +0200, Mike Galbraith wrote:
> On Fri, 2018-07-06 at 17:56 -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.
> 
> Otherwise virgin master.today grumbles.
> 
>   CC      kernel/bounds.s
>   UPD     include/generated/timeconst.h
>   UPD     include/generated/bounds.h
>   CC      arch/x86/kernel/asm-offsets.s
>   UPD     include/generated/asm-offsets.h
>   CALL    scripts/checksyscalls.sh
>   CHK     include/generated/compile.h
>   HOSTCC  usr/gen_init_cpio
>   UPD     include/generated/compile.h
>   CC      init/main.o
> In file included from ./include/linux/cpumask.h:12:0,
>                  from ./arch/x86/include/asm/cpumask.h:5,
>                  from ./arch/x86/include/asm/msr.h:11,
>                  from ./arch/x86/include/asm/processor.h:21,
>                  from ./arch/x86/include/asm/cpufeature.h:5,
>                  from ./arch/x86/include/asm/thread_info.h:53,
>                  from ./include/linux/thread_info.h:38,
>                  from ./arch/x86/include/asm/preempt.h:7,
>                  from ./include/linux/preempt.h:81,
>                  from ./include/linux/spinlock.h:51,
>                  from ./include/linux/seqlock.h:36,
>                  from ./include/linux/time.h:6,
>                  from ./include/linux/stat.h:19,
>                  from ./include/linux/module.h:10,
>                  from init/main.c:16:
> In function ‘bitmap_zero’,
>     inlined from ‘cpumask_clear’ at ./include/linux/cpumask.h:378:2,
>     inlined from ‘mm_init_cpumask’ at
> ./include/linux/mm_types.h:504:2,
>     inlined from ‘start_kernel’ at init/main.c:560:2:
> ./include/linux/bitmap.h:208:3: warning: ‘memset’ writing 64 bytes
> into a region of size 0 overflows the destination [-Wstringop-
> overflow=]
>    memset(dst, 0, len);
>    ^~~~~~~~~~~~~~~~~~~

I don't understand this one.

Inside init_mm we have this line:
        .cpu_bitmap     = { [BITS_TO_LONGS(NR_CPUS)] = 0},

which is the way the documentation suggests statically
allocated variable size arrays should be allocated 
and initialized.

How does that result in a memset of the same size,
on the same array, to throw an error like above?

What am I doing wrong?

-- 
All Rights Reversed.

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