linux-kernel - [PATCH v2 5/7] mm/sparse.c: add code comment about sub-section hotplug

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Message-Id: <20200220043316.19668-6-bhe@redhat.com>
Date:   Thu, 20 Feb 2020 12:33:14 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        richardw.yang@...ux.intel.com, david@...hat.com, osalvador@...e.de,
        dan.j.williams@...el.com, mhocko@...e.com, bhe@...hat.com,
        rppt@...ux.ibm.com, robin.murphy@....com
Subject: [PATCH v2 5/7] mm/sparse.c: add code comment about sub-section hotplug

It's helpful to note that sub-section is only supported in
SPARSEMEM_VMEMMAP case, but not in SPARSEMEM|!VMEMMAP case. Add
sentences into the code comment above sparse_add_section.

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

Signed-off-by: Baoquan He <bhe@...hat.com>
---
 mm/sparse.c | 39 ++++++++++++++++++++++-----------------
 1 file changed, 22 insertions(+), 17 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 66c497d6a229..14bff0b44e7c 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -778,6 +778,22 @@ static int clear_subsection_map(unsigned long pfn, unsigned long nr_pages)
 }
 #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 case 1, when subsection_map does not empty we will not be freeing
+ * the usage map, but still need to free the vmemmap range.
+ *
+ * For case 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)
 {
@@ -790,23 +806,7 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
 	rc = clear_subsection_map(pfn, nr_pages);
 	if(IS_ERR_VALUE((unsigned long)rc))
 		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
-	 */
+
 	if (!rc) {
 		unsigned long section_nr = pfn_to_section_nr(pfn);
 
@@ -926,6 +926,11 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
  *
  * This is only intended for hotplug.
  *
+ * Note that the added memory region is either section aligned, or
+ * sub-section aligned. The subsection hotplug is only supported in
+ * VMEMMAP case, please refer to ZONE_DEVICE part of memory-model.rst
+ * for more details.
+ *
  * Return:
  * * 0		- On success.
  * * -EEXIST	- Section has been present.
-- 
2.17.2