Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page check

[Miaohe Lin]

On 2022/4/12 16:59, Oscar Salvador wrote:
> On Sat, Apr 09, 2022 at 05:34:53PM +0800, Miaohe Lin wrote:
>> We don't need to check can_split_folio() because folio_maybe_dma_pinned()
>> is checked before. It will avoid the long term pinned pages to be swapped
>> out. And we can live with short term pinned pages. Without can_split_folio
>> checking we can simplify the code. Also activate_locked can be changed to
>> keep_locked as it's just short term pinning.
> 
> What do you mean by "we can live with short term pinned pages"?
> Does it mean that it was not pinned when we check
> folio_maybe_dma_pinned() but now it is?
> 
> To me it looks like the pinning is fluctuating and we rely on
> split_folio_to_list() to see whether we succeed or not, and if not
> we give it another spin in the next round?

Yes. Short term pinned pages is relative to long term pinned pages and these pages won't be
pinned for a noticeable time. So it's expected to split the folio successfully in the next
round as the pinning is really fluctuating. Or am I miss something?

Many thanks for your comment and reply!

> 
>> Suggested-by: Huang, Ying <ying.huang@xxxxxxxxx>
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>  mm/vmscan.c | 22 ++++++++--------------
>>  1 file changed, 8 insertions(+), 14 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 4a76be47bed1..01f5db75a507 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1711,20 +1711,14 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>>  					goto keep_locked;
>>  				if (folio_maybe_dma_pinned(folio))
>>  					goto keep_locked;
>> -				if (PageTransHuge(page)) {
>> -					/* cannot split THP, skip it */
>> -					if (!can_split_folio(folio, NULL))
>> -						goto activate_locked;
>> -					/*
>> -					 * Split pages without a PMD map right
>> -					 * away. Chances are some or all of the
>> -					 * tail pages can be freed without IO.
>> -					 */
>> -					if (!folio_entire_mapcount(folio) &&
>> -					    split_folio_to_list(folio,
>> -								page_list))
>> -						goto activate_locked;
>> -				}
>> +				/*
>> +				 * Split pages without a PMD map right
>> +				 * away. Chances are some or all of the
>> +				 * tail pages can be freed without IO.
>> +				 */
>> +				if (PageTransHuge(page) && !folio_entire_mapcount(folio) &&
>> +				    split_folio_to_list(folio, page_list))
>> +					goto keep_locked;
>>  				if (!add_to_swap(page)) {
>>  					if (!PageTransHuge(page))
>>  						goto activate_locked_split;
>> -- 
>> 2.23.0
>>
>>
>