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SubjectRe: [RESEND PATCH] mm/compaction: fix an undefined behaviour
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On 3/13/19 7:06 PM, 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")
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
> Signed-off-by: Qian Cai <cai@lca.pw>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>
> Resend because Andrew's email was bounced back at some point.
>
> mm/compaction.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index f171a83707ce..6aebf1eb8d98 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;
> }
>
> /*
>

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