The patch titled
Subject: include/linux/mmzone.h: fix explanation of lower bits in the SPARSEMEM mem_map pointer
has been added to the -mm tree. Its filename is
fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer.patch
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From: Petr Tesarik <ptesarik@xxxxxxxx>
Subject: include/linux/mmzone.h: 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.
Link: http://lkml.kernel.org/r/20180119080908.3a662e6f@xxxxxxxxxxxxxxx
Signed-off-by: Petr Tesarik <ptesarik@xxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Kemi Wang <kemi.wang@xxxxxxxxx>
Cc: YASUAKI ISHIMATSU <yasu.isimatu@xxxxxxxxx>
Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/mmzone.h | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff -puN include/linux/mmzone.h~fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer include/linux/mmzone.h
--- a/include/linux/mmzone.h~fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer
+++ a/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)
_
Patches currently in -mm which might be from ptesarik@xxxxxxxx are
fix-explanation-of-lower-bits-in-the-sparsemem-mem_map-pointer.patch
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