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

Baoquan He
2020-02-20T04:34:03Z

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@redhat.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