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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@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
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