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-rw-r--r--include/linux/mmzone.h12
1 files 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)