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

[Mel Gorman]

On Fri, Mar 08, 2019 at 05:46:50PM -0500, Qian Cai 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")
> Signed-off-by: Qian Cai <cai@xxxxxx>

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

FWIW, I had seen the same message when trying to isolate potential
corruption (still unsuccessful, the tests always complete) but
considered it relatively benign and harmless. Still worth fixing so
thanks.

-- 
Mel Gorman
SUSE Labs