[PATCH 3/3] mm/sparsemem: avoid memmap overwrite for non-SPARSEMEM_VMEMMAP

[Wei Yang]

In case of SPARSEMEM, populate_section_memmap() would allocate memmap
for the whole section, even we just want a sub-section. This would lead
to memmap overwrite if we a sub-section to an already populated section.

Just return the populated memmap for non-SPARSEMEM_VMEMMAP case.

Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
Signed-off-by: Wei Yang <richardw.yang@xxxxxxxxxxxxxxx>
CC: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 mm/sparse.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/sparse.c b/mm/sparse.c
index 56816f653588..c75ca40db513 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -836,6 +836,16 @@ static struct page * __meminit section_activate(int nid, unsigned long pfn,
 	if (nr_pages < PAGES_PER_SECTION && early_section(ms))
 		return pfn_to_page(pfn);
 
+	/*
+	 * If it is not SPARSEMEM_VMEMMAP, we always populate memmap for the
+	 * whole section, even for a sub-section.
+	 *
+	 * Return its memmap if already populated to avoid memmap overwrite.
+	 */
+	if (!IS_ENABLED(CONFIG_SPARSEMEM_VMEMMAP) &&
+		valid_section(ms))
+		return __section_mem_map_addr(ms);
+
 	memmap = populate_section_memmap(pfn, nr_pages, nid, altmap);
 	if (!memmap) {
 		section_deactivate(pfn, nr_pages, altmap);
-- 
2.17.1