[PATCH v4 4/5] mm/sparse.c: add note about only VMEMMAP supporting sub-section hotplug

[Baoquan He <bhe@redhat.com>]

And tell check_pfn_span() gating the porper alignment and size of
hot added memory region.

And also move the code comments from inside section_deactivate()
to being above it. The code comments are reasonable for the whole
function, and the moving makes code cleaner.

Signed-off-by: Baoquan He <bhe@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
---
 mm/sparse.c | 38 +++++++++++++++++++++-----------------
 1 file changed, 21 insertions(+), 17 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 117fe4554c38..f02a524e17d1 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -771,6 +771,22 @@ static bool is_subsection_map_empty(struct mem_section *ms)
 }
 #endif
 
+/*
+ * To deactivate a memory region, there are 3 cases to handle across
+ * two configurations (SPARSEMEM_VMEMMAP={y,n}):
+ *
+ * 1. deactivation of a partial hot-added section (only possible in
+ *    the SPARSEMEM_VMEMMAP=y case).
+ *      a) section was present at memory init.
+ *      b) section was hot-added post memory init.
+ * 2. deactivation of a complete hot-added section.
+ * 3. deactivation of a complete section from memory init.
+ *
+ * For 1, when subsection_map does not empty we will not be freeing the
+ * usage map, but still need to free the vmemmap range.
+ *
+ * For 2 and 3, the SPARSEMEM_VMEMMAP={y,n} cases are unified
+ */
 static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 		struct vmem_altmap *altmap)
 {
@@ -781,23 +797,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 
 	if (clear_subsection_map(pfn, nr_pages))
 		return;
-	/*
-	 * There are 3 cases to handle across two configurations
-	 * (SPARSEMEM_VMEMMAP={y,n}):
-	 *
-	 * 1/ deactivation of a partial hot-added section (only possible
-	 * in the SPARSEMEM_VMEMMAP=y case).
-	 *    a/ section was present at memory init
-	 *    b/ section was hot-added post memory init
-	 * 2/ deactivation of a complete hot-added section
-	 * 3/ deactivation of a complete section from memory init
-	 *
-	 * For 1/, when subsection_map does not empty we will not be
-	 * freeing the usage map, but still need to free the vmemmap
-	 * range.
-	 *
-	 * For 2/ and 3/ the SPARSEMEM_VMEMMAP={y,n} cases are unified
-	 */
+
 	empty = is_subsection_map_empty(ms);
 	if (empty) {
 		unsigned long section_nr = pfn_to_section_nr(pfn);
@@ -905,6 +905,10 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
  *
  * This is only intended for hotplug.
  *
+ * Note that only VMEMMAP supports sub-section aligned hotplug,
+ * the proper alignment and size are gated by check_pfn_span().
+ *
+ *
  * Return:
  * * 0		- On success.
  * * -EEXIST	- Section has been present.
-- 
2.17.2