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

[Andrew Morton]

On Wed, 20 Mar 2019 16:33:38 -0400 Qian Cai <cai@xxxxxx> wrote:

> 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@xxxxxxx>
> Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
>  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;
>  }

That's rather an eyesore.  How about

static inline unsigned int
freelist_scan_limit(struct compact_control *cc)
{
	unsigned short shift = BITS_PER_LONG - 1;

	return (COMPACT_CLUSTER_MAX >> min(shift, cc->fast_search_fail)) + 1;
}