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

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Date:   Wed, 20 Mar 2019 18:12:40 -0400
From:   Qian Cai <cai@....pw>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     mgorman@...hsingularity.net, vbabka@...e.cz, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND#2 PATCH] mm/compaction: fix an undefined behaviour



On 3/20/19 5:58 PM, Andrew Morton wrote:
>> ---
>>  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;
> }

Agree. It looks better.