linux-kernel - [PATCH] Fix explanation of lower bits in the SPARSEMEM mem

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Date:   Fri, 19 Jan 2018 08:09:08 +0100
From:   Petr Tesarik <ptesarik@...e.com>
To:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemi Wang <kemi.wang@...el.com>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Nikolay Borisov <nborisov@...e.com>
Subject: [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map
 pointer

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@...e.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