mm/page_alloc: don't call pfn_to_page() on possibly non-existent PFN in split_large_buddy()

In split_large_buddy(), we might call pfn_to_page() on a PFN that might
not exist.  In corner cases, such as when freeing the highest pageblock in
the last memory section, this could result with CONFIG_SPARSEMEM &&
!CONFIG_SPARSEMEM_EXTREME in __pfn_to_section() returning NULL and and
__section_mem_map_addr() dereferencing that NULL pointer.

Let's fix it, and avoid doing a pfn_to_page() call for the first
iteration, where we already have the page.

So far this was found by code inspection, but let's just CC stable as the
fix is easy.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: fd919a8 ("mm: page_isolation: prepare for hygienic freelists")
Signed-off-by: David Hildenbrand <[email protected]>
Reported-by: Vlastimil Babka <[email protected]>
Closes: https://lkml.kernel.org/r/[email protected]
Reviewed-by: Vlastimil Babka <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: davidhildenbrand

mm/page_alloc.c

@@ -1238,13 +1238,15 @@ static void split_large_buddy(struct zone *zone, struct page *page,
 	if (order > pageblock_order)
 		order = pageblock_order;
 
-	while (pfn != end) {
+	do {
 		int mt = get_pfnblock_migratetype(page, pfn);
 
 		__free_one_page(page, pfn, zone, order, mt, fpi);
 		pfn += 1 << order;
+		if (pfn == end)
+			break;
 		page = pfn_to_page(pfn);
-	}
+	} while (1);
 }
 
 static void free_one_page(struct zone *zone, struct page *page,