[PATCH v4 2/5] mm: Teach pfn_to_online_page() to consider subsection validity

[Dan Williams]

pfn_section_valid() determines pfn validity on subsection granularity
where pfn_valid() may be limited to coarse section granularity.
Explicitly validate subsections after pfn_valid() succeeds.

Fixes: b13bc35193d9 ("mm/hotplug: invalid PFNs from pfn_to_online_page()")
Cc: Qian Cai <cai@xxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Reported-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 mm/memory_hotplug.c |   23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 55a69d4396e7..d0c81f7a3347 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -308,11 +308,26 @@ static int check_hotplug_memory_addressable(unsigned long pfn,
 struct page *pfn_to_online_page(unsigned long pfn)
 {
 	unsigned long nr = pfn_to_section_nr(pfn);
+	struct mem_section *ms;
+
+	if (nr >= NR_MEM_SECTIONS)
+		return NULL;
+
+	ms = __nr_to_section(nr);
+	if (!online_section(ms))
+		return NULL;
+
+	/*
+	 * Save some code text when online_section() +
+	 * pfn_section_valid() are sufficient.
+	 */
+	if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) && !pfn_valid(pfn))
+		return NULL;
+
+	if (!pfn_section_valid(ms, pfn))
+		return NULL;
 
-	if (nr < NR_MEM_SECTIONS && online_section_nr(nr) &&
-	    pfn_valid_within(pfn))
-		return pfn_to_page(pfn);
-	return NULL;
+	return pfn_to_page(pfn);
 }
 EXPORT_SYMBOL_GPL(pfn_to_online_page);