[RESEND#2 PATCH] mm/compaction: fix an undefined behaviour

[Qian Cai <cai@lca.pw>]

In a low-memory situation, cc->fast_search_fail can keep increasing as
it is unable to find an available page to isolate in
fast_isolate_freepages(). As the result, it could trigger an error
below, so just compare with the maximum bits can be shifted first.

UBSAN: Undefined behaviour in mm/compaction.c:1160:30
shift exponent 64 is too large for 64-bit type 'unsigned long'
CPU: 131 PID: 1308 Comm: kcompactd1 Kdump: loaded Tainted: G
W    L    5.0.0+ #17
Call trace:
 dump_backtrace+0x0/0x450
 show_stack+0x20/0x2c
 dump_stack+0xc8/0x14c
 __ubsan_handle_shift_out_of_bounds+0x7e8/0x8c4
 compaction_alloc+0x2344/0x2484
 unmap_and_move+0xdc/0x1dbc
 migrate_pages+0x274/0x1310
 compact_zone+0x26ec/0x43bc
 kcompactd+0x15b8/0x1a24
 kthread+0x374/0x390
 ret_from_fork+0x10/0x18

Fixes: 70b44595eafe ("mm, compaction: use free lists to quickly locate a migration source")
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Qian Cai <cai@lca.pw>
---
 mm/compaction.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index e1a08fc92353..0d1156578114 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1157,7 +1157,9 @@ static bool suitable_migration_target(struct compact_control *cc,
 static inline unsigned int
 freelist_scan_limit(struct compact_control *cc)
 {
-	return (COMPACT_CLUSTER_MAX >> cc->fast_search_fail) + 1;
+	return (COMPACT_CLUSTER_MAX >>
+		min((unsigned short)(BITS_PER_LONG - 1), cc->fast_search_fail))
+		+ 1;
 }
 
 /*
-- 
2.17.2 (Apple Git-113)