Re: [PATCH] mm: vmscan: correct nr_reclaimed for THP

[Huang\, Ying]

Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:

> On 5/9/19 7:12 PM, Huang, Ying wrote:
>> Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> writes:
>>
>>> Since commit bd4c82c22c36 ("mm, THP, swap: delay splitting THP after
>>> swapped out"), THP can be swapped out in a whole.  But, nr_reclaimed
>>> still gets inc'ed by one even though a whole THP (512 pages) gets
>>> swapped out.
>>>
>>> This doesn't make too much sense to memory reclaim.  For example, direct
>>> reclaim may just need reclaim SWAP_CLUSTER_MAX pages, reclaiming one THP
>>> could fulfill it.  But, if nr_reclaimed is not increased correctly,
>>> direct reclaim may just waste time to reclaim more pages,
>>> SWAP_CLUSTER_MAX * 512 pages in worst case.
>>>
>>> This change may result in more reclaimed pages than scanned pages showed
>>> by /proc/vmstat since scanning one head page would reclaim 512 base pages.
>>>
>>> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
>>> Cc: Michal Hocko <mhocko@xxxxxxxx>
>>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
>>> Cc: "Kirill A . Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>>> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
>>> ---
>>> I'm not quite sure if it was the intended behavior or just omission. I tried
>>> to dig into the review history, but didn't find any clue. I may miss some
>>> discussion.
>>>
>>>   mm/vmscan.c | 6 +++++-
>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index fd9de50..7e026ec 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -1446,7 +1446,11 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>>     		unlock_page(page);
>>>   free_it:
>>> -		nr_reclaimed++;
>>> +		/*
>>> +		 * THP may get swapped out in a whole, need account
>>> +		 * all base pages.
>>> +		 */
>>> +		nr_reclaimed += (1 << compound_order(page));
>>>     		/*
>>>   		 * Is there need to periodically free_page_list? It would
>> Good catch!  Thanks!
>>
>> How about to change this to
>>
>>
>>          nr_reclaimed += hpage_nr_pages(page);
>
> Either is fine to me. Is this faster than "1 << compound_order(page)"?

I think the readability is a little better.  And this will become

        nr_reclaimed += 1

if CONFIG_TRANSPARENT_HUAGEPAGE is disabled.

Best Regards,
Huang, Ying

>>
>> Best Regards,
>> Huang, Ying