[Crash-utility] [PATCH] Update for the "kmem -n" option on Linux 5.3-rc1 and later kernels

[Kazuhito Hagio]

Update for the "kmem -n" option on Linux 5.3-rc1 and later kernels that
contain commit 326e1b8f83a4318b09033ef754f40c785aed5e68, titled
"mm/sparsemem: introduce a SECTION_IS_EARLY flag". With this patch,
the option displays a new state "E" for the flag, instead of printing
incorrect mem_map addresses.

Also update the confusing comment with the one from kernel commit
def9b71ee651a6fee93a10734b94f93a69cdb2d4, titled "include/linux/mmzone.h:
fix explanation of lower bits in the SPARSEMEM mem_map pointer".

Signed-off-by: Kazuhito Hagio <k-hagio at ab.jp.nec.com>
---
 help.c   |  4 ++--
 memory.c | 17 ++++++++++++++---
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/help.c b/help.c
index d5940f789ffd..a5218a701c01 100644
--- a/help.c
+++ b/help.c
@@ -6597,8 +6597,8 @@ char *help_kmem[] = {
 "            the cumulative, vm_event_states counter values if they exist.",
 "        -n  display memory node, memory section, and memory block data",
 "            and state; the state of each memory section state is encoded",
-"            as \"P\", \"M\" and/or \"O\", meaning SECTION_MARKED_PRESENT,",
-"            SECTION_HAS_MEM_MAP and SECTION_IS_ONLINE.",
+"            as \"P\", \"M\", \"O\" and/or \"E\", meaning SECTION_MARKED_PRESENT,",
+"            SECTION_HAS_MEM_MAP, SECTION_IS_ONLINE and SECTION_IS_EARLY.",
 "        -z  displays per-zone memory statistics.",
 "        -o  displays each cpu's offset value that is added to per-cpu symbol",
 "            values to translate them into kernel virtual addresses.",
diff --git a/memory.c b/memory.c
index 327e5355ec9c..ac7e73679405 100644
--- a/memory.c
+++ b/memory.c
@@ -17154,13 +17154,22 @@ nr_to_section(ulong nr)
 
 /*
  * 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)
 #define SECTION_IS_ONLINE	(1UL<<2)
-#define SECTION_MAP_LAST_BIT	(1UL<<3)
+#define SECTION_IS_EARLY	(1UL<<3)
+#define SECTION_MAP_LAST_BIT	(1UL<<4)
 #define SECTION_MAP_MASK	(~(SECTION_MAP_LAST_BIT-1))
 
 
@@ -17257,6 +17266,8 @@ fill_mem_section_state(ulong state, char *buf)
 		bufidx += sprintf(buf + bufidx, "%s", "M");
 	if (state & SECTION_IS_ONLINE)
 		bufidx += sprintf(buf + bufidx, "%s", "O");
+	if (state & SECTION_IS_EARLY)
+		bufidx += sprintf(buf + bufidx, "%s", "E");
 }
 
 void 
-- 
2.18.1