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Subject: Re: [PATCH 2/3] mm/sparse: expand the CONFIG_SPARSEMEM_EXTREME range
 in __nr_to_section()
References: <20180823130732.9489-1-richard.weiyang@gmail.com>
 <20180823130732.9489-3-richard.weiyang@gmail.com>
 <20180823132112.GK29735@dhcp22.suse.cz>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <ebdfe4aa-225c-8239-9f8d-065de8a5ddfc@intel.com>
Date: Thu, 23 Aug 2018 17:09:12 -0700
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Sender: owner-linux-mm@kvack.org
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To: Michal Hocko <mhocko@suse.com>, Wei Yang <richard.weiyang@gmail.com>
Cc: akpm@linux-foundation.org, rientjes@google.com, linux-mm@kvack.org, kirill.shutemov@linux.intel.com, bob.picco@hp.com

On 08/23/2018 06:21 AM, Michal Hocko wrote:
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1155,9 +1155,9 @@ static inline struct mem_section *__nr_to_section(unsigned long nr)
>  #ifdef CONFIG_SPARSEMEM_EXTREME
>  	if (!mem_section)
>  		return NULL;
> -#endif
>  	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
>  		return NULL;
> +#endif
>  	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
>  }

This patch has no practical effect and only adds unnecessary churn.

#ifdef CONFIG_SPARSEMEM_EXTREME
...
#else
struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
#endif

The compiler knows that NR_SECTION_ROOTS==1 and that
!mem_section[SECTION_NR_TO_ROOT(nr) is always false.  It doesn't need
our help.

My goal with the sparsemem code, and code in general is t avoid #ifdefs
whenever possible and limit their scope to the smallest possible area
whenever possible.