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Subject[PATCH v3 5/7] mm/sparse.c: add note about only VMEMMAP supporting sub-section support
Date
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>
---
mm/sparse.c | 37 ++++++++++++++++++++-----------------
1 file changed, 20 insertions(+), 17 deletions(-)

diff --git a/mm/sparse.c b/mm/sparse.c
index 2142045ab5c5..0fbd79c4ad81 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -772,6 +772,22 @@ 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)
{
@@ -784,23 +800,6 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages,
return;

empty = is_subsection_map_empty(ms);
- /*
- * 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 (empty) {
unsigned long section_nr = pfn_to_section_nr(pfn);

@@ -907,6 +906,10 @@ 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
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