hugetlbfs: don't access uninitialized memmaps in pfn_range_valid_gigantic()

Uninitialized memmaps contain garbage and in the worst case trigger
kernel BUGs, especially with CONFIG_PAGE_POISONING.  They should not get
touched.

Let's make sure that we only consider online memory (managed by the
buddy) that has initialized memmaps.  ZONE_DEVICE is not applicable.

page_zone() will call page_to_nid(), which will trigger
VM_BUG_ON_PGFLAGS(PagePoisoned(page), page) with CONFIG_PAGE_POISONING
and CONFIG_DEBUG_VM_PGFLAGS when called on uninitialized memmaps.  This
can be the case when an offline memory block (e.g., never onlined) is
spanned by a zone.

Note: As explained by Michal in [1], alloc_contig_range() will verify
the range.  So it boils down to the wrong access in this function.

[1] http://lkml.kernel.org/r/[email protected]

Link: http://lkml.kernel.org/r/[email protected]
Fixes: f1dd2cd ("mm, memory_hotplug: do not associate hotadded memory to zones until online")	[visible after d0dc12e]
Signed-off-by: David Hildenbrand <[email protected]>
Reported-by: Michal Hocko <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: <[email protected]>	[4.13+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: davidhildenbrand

mm/hugetlb.c

@@ -1084,11 +1084,10 @@ static bool pfn_range_valid_gigantic(struct zone *z,
 	struct page *page;
 
 	for (i = start_pfn; i < end_pfn; i++) {
-		if (!pfn_valid(i))
+		page = pfn_to_online_page(i);
+		if (!page)
 			return false;
 
-		page = pfn_to_page(i);
-
 		if (page_zone(page) != z)
 			return false;