mm: avoid passing 0 to __ffs()

23baf831a32c ("mm, treewide: redefine MAX_ORDER sanely") results in
various boot failures (hang) on arm targets Debug messages reveal the
reason.

########### MAX_ORDER=10 start=0 __ffs(start)=-1 min()=10 min_t=-1
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If start==0, __ffs(start) returns 0xfffffff or (as int) -1, which min_t()
interprets as such, while min() apparently uses the returned unsigned long
value. Obviously a negative order isn't received well by the rest of the
code.

[[email protected]: fix comment, per Mike]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 23baf831a32c ("mm, treewide: redefine MAX_ORDER sanely")
Signed-off-by: "Kirill A. Shutemov" <[email protected]>
Reported-by: Guenter Roeck <[email protected]>
  Link: https://lkml.kernel.org/r/[email protected]
Reviewed-by: Mike Rapoport (IBM) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

2 files changed, 22 insertions, 2 deletions

diff --git a/mm/memblock.c b/mm/memblock.c
index 7911224b1ed3..3feafea06ab2 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -2043,7 +2043,16 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end)
 	int order;
 
 	while (start < end) {
-		order = min_t(int, MAX_ORDER, __ffs(start));
+		/*
+		 * Free the pages in the largest chunks alignment allows.
+		 *
+		 * __ffs() behaviour is undefined for 0. start == 0 is
+		 * MAX_ORDER-aligned, set order to MAX_ORDER for the case.
+		 */
+		if (start)
+			order = min_t(int, MAX_ORDER, __ffs(start));
+		else
+			order = MAX_ORDER;
 
 		while (start + (1UL << order) > end)
 			order--;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index c8f0a8c2d049..8e0fa209d533 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -605,7 +605,18 @@ static void online_pages_range(unsigned long start_pfn, unsigned long nr_pages)
 	 * this and the first chunk to online will be pageblock_nr_pages.
 	 */
 	for (pfn = start_pfn; pfn < end_pfn;) {
-		int order = min_t(int, MAX_ORDER, __ffs(pfn));
+		int order;
+
+		/*
+		 * Free to online pages in the largest chunks alignment allows.
+		 *
+		 * __ffs() behaviour is undefined for 0. start == 0 is
+		 * MAX_ORDER-aligned, Set order to MAX_ORDER for the case.
+		 */
+		if (pfn)
+			order = min_t(int, MAX_ORDER, __ffs(pfn));
+		else
+			order = MAX_ORDER;
 
 		(*online_page_callback)(pfn_to_page(pfn), order);
 		pfn += (1UL << order);