From: Petr Tesarik <ptesarik@suse.com> To: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz> Cc: 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: [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map pointer Date: Fri, 19 Jan 2018 08:09:08 +0100 Message-ID: <20180119080908.3a662e6f@ezekiel.suse.cz> (raw) The comment is confusing. On the one hand, it refers to 32-bit alignment (struct page alignment on 32-bit platforms), but this would only guarantee that the 2 lowest bits must be zero. On the other hand, it claims that at least 3 bits are available, and 3 bits are actually used. This is not broken, because there is a stronger alignment guarantee, just less obvious. Let's fix the comment to make it clear how many bits are available and why. Signed-off-by: Petr Tesarik <ptesarik@suse.com> --- include/linux/mmzone.h | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) 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. */ #define SECTION_MARKED_PRESENT (1UL<<0) #define SECTION_HAS_MEM_MAP (1UL<<1) -- 2.13.6 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next reply index Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-01-19 7:09 Petr Tesarik [this message] 2018-01-19 12:39 ` Michal Hocko 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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