Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[Ryan Roberts]

On 11/03/2024 22:30, Barry Song wrote:
> On Mon, Mar 11, 2024 at 11:01 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> Now that swap supports storing all mTHP sizes, avoid splitting large
>> folios before swap-out. This benefits performance of the swap-out path
>> by eliding split_folio_to_list(), which is expensive, and also sets us
>> up for swapping in large folios in a future series.
>>
>> If the folio is partially mapped, we continue to split it since we want
>> to avoid the extra IO overhead and storage of writing out pages
>> uneccessarily.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>  mm/vmscan.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index cf7d4cf47f1a..0ebec99e04c6 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>                                         if (!can_split_folio(folio, NULL))
>>                                                 goto activate_locked;
>>                                         /*
>> -                                        * Split folios without a PMD map right
>> -                                        * away. Chances are some or all of the
>> -                                        * tail pages can be freed without IO.
>> +                                        * Split partially mapped folios map
>> +                                        * right away. Chances are some or all
>> +                                        * of the tail pages can be freed
>> +                                        * without IO.
>>                                          */
>> -                                       if (!folio_entire_mapcount(folio) &&
>> +                                       if (!list_empty(&folio->_deferred_list) &&
> 
> Hi Ryan,
> After reconsidering our previous discussion about PMD-mapped large
> folios, I've pondered
> the possibility of PMD-mapped Transparent Huge Pages (THPs) being
> mapped by multiple
> processes. In such a scenario, if one process decides to unmap a
> portion of the folio while
> others retain the entire mapping, it raises questions about how the
> system should handle
> this situation. Would the large folio be placed in a deferred list? 

No - if the large folio is entirely mapped (via PMD), then the folio will not be
put on the deferred split list in the first place. See __folio_remove_rmap():

	last = (last < ENTIRELY_MAPPED);

means that nr will never be incremented above 0. (_nr_pages_mapped is
incremented by ENTIRELY_MAPPED for every PMD map).

> If
> so, splitting it might not
> yield benefits, as neither I/O nor swap slots would increase in this
> case by not splitting it.
> 
> Regarding PTE-mapped large folios, the absence of an indicator like
> "entire_map" makes it
> challenging to identify cases where the entire folio is mapped. Thus,
> splitting seems to be
> the only viable solution in such circumstances.
> 
>>                                             split_folio_to_list(folio,
>>                                                                 folio_list))
>>                                                 goto activate_locked;
>> --
>> 2.25.1
> 
> Thanks
> Barry