Re: [PATCH v5 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[Ryan Roberts]

On 28/03/2024 08:18, Barry Song wrote:
> On Thu, Mar 28, 2024 at 3:45 AM 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.
>>
>> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
>> Reviewed-by: Barry Song <v-songbaohua@xxxxxxxx>
>> 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 00adaf1cb2c3..293120fe54f3 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1223,11 +1223,12 @@ 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 right
>> +                                        * away. We can free the unmapped pages
>> +                                        * without IO.
>>                                          */
>> -                                       if (!folio_entire_mapcount(folio) &&
>> +                                       if (data_race(!list_empty(
>> +                                               &folio->_deferred_list)) &&
>>                                             split_folio_to_list(folio,
>>                                                                 folio_list))
>>                                                 goto activate_locked;
> 
> Hi Ryan,
> 
> Sorry for bringing up another minor issue at this late stage.

No problem - I'd rather take a bit longer and get it right, rather than rush it
and get it wrong!

> 
> During the debugging of thp counter patch v2, I noticed the discrepancy between
> THP_SWPOUT_FALLBACK and THP_SWPOUT.
> 
> Should we make adjustments to the counter?

Yes, agreed; we want to be consistent here with all the other existing THP
counters; they only refer to PMD-sized THP. I'll make the change for the next
version.

I guess we will eventually want equivalent counters for per-size mTHP using the
framework you are adding.

> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 293120fe54f3..d7856603f689 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1241,8 +1241,10 @@ static unsigned int shrink_folio_list(struct
> list_head *folio_list,
>                                                                 folio_list))
>                                                 goto activate_locked;
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -
> count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
> -                                       count_vm_event(THP_SWPOUT_FALLBACK);
> +                                       if (folio_test_pmd_mappable(folio)) {
> +
> count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
> +
> count_vm_event(THP_SWPOUT_FALLBACK);
> +                                       }
>  #endif
>                                         if (!add_to_swap(folio))
>                                                 goto activate_locked_split;
> 
> 
> Because THP_SWPOUT is only for pmd:
> 
> static inline void count_swpout_vm_event(struct folio *folio)
> {
> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         if (unlikely(folio_test_pmd_mappable(folio))) {
>                 count_memcg_folio_events(folio, THP_SWPOUT, 1);
>                 count_vm_event(THP_SWPOUT);
>         }
> #endif
>         count_vm_events(PSWPOUT, folio_nr_pages(folio));
> }
> 
> I can provide per-order counters for this in my THP counter patch.
> 
>> --
>> 2.25.1
>>
> 
> Thanks
> Barry