[061/166] mm/sparse.c: add note about only VMEMMAP supporting sub-section hotplug

Commit Message

Andrew Morton April 7, 2020, 3:07 a.m. UTC

From: Baoquan He <[e-mail]>
Subject: mm/sparse.c: add note about only VMEMMAP supporting sub-section hotplug

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.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Baoquan He <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Pankaj Gupta <[e-mail]>
Cc: Wei Yang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/sparse.c |   38 +++++++++++++++++++++-----------------
 1 file changed, 21 insertions(+), 17 deletions(-)

Patch

--- a/mm/sparse.c~mm-sparsec-add-note-about-only-vmemmap-supporting-sub-section-hotplug
+++ a/mm/sparse.c
@@ -750,6 +750,22 @@  static bool is_subsection_map_empty(stru
 }
 #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)
 {
@@ -760,23 +776,7 @@  static void section_deactivate(unsigned
 
 	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);
@@ -890,6 +890,10 @@  static struct page * __meminit section_a
  *
  * 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.