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

[Barry Song <21cnbao@gmail.com>]

On Thu, Mar 28, 2024 at 3:45 AM Ryan Roberts <ryan.roberts@arm.com> 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@redhat.com>
> Reviewed-by: Barry Song <v-songbaohua@oppo.com>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
>  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.

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?

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