Re: [PATCH v2] mm/compaction: remove unnecessary detection code.

[Kemeng Shi <shikemeng@huaweicloud.com>]

Hello
on 11/14/2024 3:44 PM, Vlastimil Babka wrote:
> On 11/14/24 07:57, Qiang Liu wrote:
>> It is impossible for the situation where blockpfn > end_pfn to arise,
>> The if statement here is not only unnecessary, but may also lead to
>> a misunderstanding that blockpfn > end_pfn could potentially happen.
>> so these unnecessary checking code should be removed.
>>
>> Signed-off-by: Qiang Liu <liuq131@chinatelecom.cn>
> 
As stride could 32, if isolate_freepages_range() is called with start_pfn not
aligned with 32, we could bail out look with blockpfn > end_pfn in
isolate_freepages_block(). Please correct if I miss something.
> I see that's since 3da0272a4c7d ("mm/compaction: correctly return failure
> with bogus compound_order in strict mode")
> 
> I think that commit introduced a risk of overflow due to a bogus order
> (which we read in a racy way), and once blockpfn overflows it will satisfy
> <= end_pfn and might e.g. end up scanning a completely different zone?
> 
>                         if (blockpfn + (1UL << order) <= end_pfn) {
> 
>                                 blockpfn += (1UL << order) - 1;
>                                 page += (1UL << order) - 1;
>                                 nr_scanned += (1UL << order) - 1;
>                         }
> 
> We should better add back the MAX_ORDER sanity check?
As order of pageblock is <= MAX_ORDER, if bogus order is > MAX_ORDER, then
blockpfn + (1UL << order) must be > end_pfn, I think the sanity check is
not needed.

Thanks.
Kemeng
> 
>> ---
>>  mm/compaction.c | 6 ------
>>  1 file changed, 6 deletions(-)
>>
>> diff --git a/mm/compaction.c b/mm/compaction.c
>> index a2b16b08cbbf..baeda7132252 100644
>> --- a/mm/compaction.c
>> +++ b/mm/compaction.c
>> @@ -682,12 +682,6 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>>  	if (locked)
>>  		spin_unlock_irqrestore(&cc->zone->lock, flags);
>>  
>> -	/*
>> -	 * Be careful to not go outside of the pageblock.
>> -	 */
>> -	if (unlikely(blockpfn > end_pfn))
>> -		blockpfn = end_pfn;
>> -
>>  	trace_mm_compaction_isolate_freepages(*start_pfn, blockpfn,
>>  					nr_scanned, total_isolated);
>>  
>