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Subject: Re: [PATCH] sparse: Track the boundaries of memory sections for
 accurate checks
References: <1466244679-23824-1-git-send-email-karahmed@amazon.de>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <57D9E5E2.7060405@intel.com>
Date: Wed, 14 Sep 2016 17:05:54 -0700
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Sender: owner-linux-mm@kvack.org
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To: KarimAllah Ahmed <karahmed@amazon.de>, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@techsingularity.net>, Vlastimil Babka <vbabka@suse.cz>, Michal Hocko <mhocko@suse.com>, Johannes Weiner <hannes@cmpxchg.org>, Yaowei Bai <baiyaowei@cmss.chinamobile.com>, Dan Williams <dan.j.williams@intel.com>, Joe Perches <joe@perches.com>, Tejun Heo <tj@kernel.org>, Anthony Liguori <aliguori@amazon.com>, "=?UTF-8?Q?Jan_H_._Sch=c3=b6nherr?=" <jschoenh@amazon.de>

On 06/18/2016 03:11 AM, KarimAllah Ahmed wrote:
> @@ -1067,8 +1067,12 @@ struct mem_section {
>  	 * section. (see page_ext.h about this.)
>  	 */
>  	struct page_ext *page_ext;
> -	unsigned long pad;
> +	unsigned long pad[3];
>  #endif
> +
> +	unsigned long first_pfn;
> +	unsigned long last_pfn;

mem_section started out as a single pointer, and it's getting a bit, um,
rotund.  Remember, some architectures have a lot of physical address
space, and thus a ton of mem_sections, but very little actual memory.
This eats valuable RAM if we bloat mem_section too much.

Oh, and with this:

>         /*
>          * WARNING: mem_section must be a power-of-2 in size for the
>          * calculation and use of SECTION_ROOT_MASK to make sense.
>          */

Aren't you making a non-power-of-2 sized 'mem_section" in some cases?

This also doesn't handle if there is a hole in the _middle_ of a section.

What's wrong with using some bits in the existing usemap?

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