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 [thread overview]
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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next prev parent reply other threads:[~2018-01-19 12:40 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-19 7:09 [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map pointer 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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