mm/compaction.c: fix an undefined behaviour

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

Change-Id: I3e4d978adff2be903ee85ae2668a557eadc71ad2
Link: http://lkml.kernel.org/r/20190320203338.53367-1-cai@lca.pw


Fixes: 70b44595eafe ("mm, compaction: use free lists to quickly locate a migration source")
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Git-commit: b6643a56f0e5958ee33affdcfc06eb7d88557a79
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git


Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>