[PATCH] mm/sparse: set SECTION_NID_SHIFT to 6

[Naoya Horiguchi]

Hagio-san reported that crash utility can see bit 4 in section_mem_map
(SECTION_TAINT_ZONE_DEVICE) to be set, even if we do not use any
ZONE_DEVICE ilke pmem or HMM.  This problem could break crash-related
toolsets and/or other memory analysis tools.

The root cause is that SECTION_NID_SHIFT is incorrectly set to 3,
while we use lower 5 bits for SECTION_* flags.  So bit 3 and 4 can be
overlapped by sub-field for early NID, and bit 4 is unexpectedly set
on (for example) NUMA node id is 2 or 3.

To fix it, set SECTION_NID_SHIFT to 6 which is the minimum number of
available bits of section flag field.

[1]: https://github.com/crash-utility/crash/commit/0b5435e10161345cf713ed447a155a611a1b408b

Fixes: 1f90a3477df3 ("mm: teach pfn_to_online_page() about ZONE_DEVICE section collisions")
Cc: stable@xxxxxxxxxxxxxxx # v5.12+
Reported-by: Kazuhito Hagio <k-hagio-ab@xxxxxxx>
Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
---
 include/linux/mmzone.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index fcb535560028..d6aa2a196aeb 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1357,6 +1357,7 @@ extern size_t mem_section_usage_size(void);
  *      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.
+ * SECTION_NID_SHIFT is set to 6 based on this fact.
  */
 #define SECTION_MARKED_PRESENT		(1UL<<0)
 #define SECTION_HAS_MEM_MAP		(1UL<<1)
@@ -1365,7 +1366,7 @@ extern size_t mem_section_usage_size(void);
 #define SECTION_TAINT_ZONE_DEVICE	(1UL<<4)
 #define SECTION_MAP_LAST_BIT		(1UL<<5)
 #define SECTION_MAP_MASK		(~(SECTION_MAP_LAST_BIT-1))
-#define SECTION_NID_SHIFT		3
+#define SECTION_NID_SHIFT		6
 
 static inline struct page *__section_mem_map_addr(struct mem_section *section)
 {
-- 
2.25.1