linux-kernel - Re: [RFC] mm: memblock: change default cnt for regions from 1 to 0

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Message-Id: <20141023121840.f88439912f23a3c2a01eb54f@linux-foundation.org>
Date:	Thu, 23 Oct 2014 12:18:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>
Cc:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Tang Chen <tangchen@...fujitsu.com>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [RFC] mm: memblock: change default cnt for regions from 1 to 0

(cc's added)

On Thu, 23 Oct 2014 17:56:53 +0100 Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com> wrote:

> The default region counts are set to 1 with a comment saying empty
> dummy entry.
> 
> If this is a dummy entry, should this be changed to 0?
> 
> We have faced this in mips/kernel/setup.c arch_mem_init.
> 
> cma uses memblock. But even with cma disabled.
> The for_each_memblock(reserved, reg) goes inside the loop.
> Even without any reserved regions.
> 
> Traced it to the following, when the macro
> for_each_memblock(memblock_type, region) is used.
> 
> It expands to add the cnt variable.
> 
> for (region = memblock.memblock_type.regions; 		\
> 	region < (memblock.memblock_type.regions + memblock.memblock_type.cnt); \
> 	region++)
> 
> In the corner case, that there are no reserved regions.
> Due to the default 1 value of cnt.
> The loop under for_each_memblock still runs once.
> 
> Even when there is no reserved region.
> 
> Is this by design? or unintentional?
> It might be that this loop runs an extra time every instance out there?
> ---
>  mm/memblock.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/memblock.c b/mm/memblock.c
> index 6d2f219..b91301c 100644
> --- a/mm/memblock.c
> +++ b/mm/memblock.c
> @@ -33,16 +33,16 @@ static struct memblock_region memblock_physmem_init_regions[INIT_PHYSMEM_REGIONS
>  
>  struct memblock memblock __initdata_memblock = {
>  	.memory.regions		= memblock_memory_init_regions,
> -	.memory.cnt		= 1,	/* empty dummy entry */
> +	.memory.cnt		= 0,	/* empty dummy entry */
>  	.memory.max		= INIT_MEMBLOCK_REGIONS,
>  
>  	.reserved.regions	= memblock_reserved_init_regions,
> -	.reserved.cnt		= 1,	/* empty dummy entry */
> +	.reserved.cnt		= 0,	/* empty dummy entry */
>  	.reserved.max		= INIT_MEMBLOCK_REGIONS,
>  
>  #ifdef CONFIG_HAVE_MEMBLOCK_PHYS_MAP
>  	.physmem.regions	= memblock_physmem_init_regions,
> -	.physmem.cnt		= 1,	/* empty dummy entry */
> +	.physmem.cnt		= 0,	/* empty dummy entry */
>  	.physmem.max		= INIT_PHYSMEM_REGIONS,
>  #endif
>  
> -- 
> 1.9.1
--
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