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From: Michal Hocko <mhocko@kernel.org>
To: Petr Tesarik <ptesarik@suse.com>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	linux-kernel@vger.kernel.org,
	Mel Gorman <mgorman@techsingularity.net>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Kemi Wang <kemi.wang@intel.com>,
	YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Nikolay Borisov <nborisov@suse.com>
Subject: Re: [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map pointer
Date: Fri, 19 Jan 2018 13:39:56 +0100
Message-ID: <20180119123956.GZ6584@dhcp22.suse.cz> (raw)
In-Reply-To: <20180119080908.3a662e6f@ezekiel.suse.cz>

On Fri 19-01-18 08:09:08, Petr Tesarik wrote:
[...]
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 67f2e3c38939..7522a6987595 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1166,8 +1166,16 @@ extern unsigned long usemap_size(void);
>  
>  /*
>   * We use the lower bits of the mem_map pointer to store
> - * a little bit of information.  There should be at least
> - * 3 bits here due to 32-bit alignment.
> + * a little bit of information.  The pointer is calculated
> + * as mem_map - section_nr_to_pfn(pnum).  The result is
> + * aligned to the minimum alignment of the two values:
> + *   1. All mem_map arrays are page-aligned.
> + *   2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT
> + *      lowest bits.  PFN_SECTION_SHIFT is arch-specific
> + *      (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
> + *      worst combination is powerpc with 256k pages,
> + *      which results in PFN_SECTION_SHIFT equal 6.
> + * To sum it up, at least 6 bits are available.
>   */

This is _much_ better indeed. Do you think we can go one step further
and add BUG_ON into the sparse code to guarantee that every mmemap
is indeed aligned properly so that SECTION_MAP_LAST_BIT-1 bits are never
used?

Thanks!

>  #define	SECTION_MARKED_PRESENT	(1UL<<0)
>  #define SECTION_HAS_MEM_MAP	(1UL<<1)
> -- 
> 2.13.6

-- 
Michal Hocko
SUSE Labs

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Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-19  7:09 Petr Tesarik
2018-01-19 12:39 ` Michal Hocko [this message]
2018-01-19 13:21   ` Petr Tesarik
2018-01-24 12:43     ` Michal Hocko
2018-01-24 13:57       ` Petr Tesarik
2018-01-25  9:05         ` [PATCH v2] " Petr Tesarik
2018-01-25 11:46           ` Michal Hocko

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