mm/page_alloc: skip non present sections on zone initialization

memmap_init_zone() can be called on the ranges with holes during the
boot.  It will skip any non-valid PFNs one-by-one.  It works fine as
long as holes are not too big.

But huge holes in the memory map causes a problem.  It takes over 20
seconds to walk 32TiB hole.  x86-64 with 5-level paging allows for much
larger holes in the memory map which would practically hang the system.

Deferred struct page init doesn't help here.  It only works on the
present ranges.

Skipping non-present sections would fix the issue.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Kirill A. Shutemov <[email protected]>
Reviewed-by: Baoquan He <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: "Jin, Zhi" <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_alloc.c

@@ -5848,6 +5848,30 @@ overlap_memmap_init(unsigned long zone, unsigned long *pfn)
 	return false;
 }
 
+#ifdef CONFIG_SPARSEMEM
+/* Skip PFNs that belong to non-present sections */
+static inline __meminit unsigned long next_pfn(unsigned long pfn)
+{
+	unsigned long section_nr;
+
+	section_nr = pfn_to_section_nr(++pfn);
+	if (present_section_nr(section_nr))
+		return pfn;
+
+	while (++section_nr <= __highest_present_section_nr) {
+		if (present_section_nr(section_nr))
+			return section_nr_to_pfn(section_nr);
+	}
+
+	return -1;
+}
+#else
+static inline __meminit unsigned long next_pfn(unsigned long pfn)
+{
+	return pfn++;
+}
+#endif
+
 /*
  * Initially all pages are reserved - free ones are freed
  * up by memblock_free_all() once the early boot process is
@@ -5887,8 +5911,10 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
 		 * function.  They do not exist on hotplugged memory.
 		 */
 		if (context == MEMMAP_EARLY) {
-			if (!early_pfn_valid(pfn))
+			if (!early_pfn_valid(pfn)) {
+				pfn = next_pfn(pfn) - 1;
 				continue;
+			}
 			if (!early_pfn_in_nid(pfn, nid))
 				continue;
 			if (overlap_memmap_init(zone, &pfn))