pfn_section_valid() determines pfn validity on subsection granularity.

pfn_valid_within() internally uses pfn_section_valid(), but gates it
with early_section() to preserve the traditional behavior of pfn_valid()
before subsection support was added.

pfn_to_online_page() wants the explicit precision that pfn_valid() does
not offer, so use pfn_section_valid() directly. Since
pfn_to_online_page() already open codes the validity of the section
number vs NR_MEM_SECTIONS, there's not much value to using
pfn_valid_within(), just use pfn_section_valid(). This loses the
valid_section() check that pfn_valid_within() was performing, but that
was already redundant with the online check.

Fixes: b13bc35193d9 ("mm/hotplug: invalid PFNs from pfn_to_online_page()")
Cc: Qian Cai <redacted>
Cc: Michal Hocko <mhocko@suse.com>
Reported-by: David Hildenbrand <redacted>
---
 mm/memory_hotplug.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 55a69d4396e7..a845b3979bc0 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c

@@ -308,11 +308,19 @@ 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;
+
+	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);