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

[Lance Yang]

Baolin Wang's patch[1] has avoided confusion with PMD mapped THP
related statistics.

So, these three counters (THP_SPLIT_PAGE, THP_SPLIT_PAGE_FAILED,
and THP_DEFERRED_SPLIT_PAGE) no longer include mTHP.

[1] https://lore.kernel.org/linux-mm/a5341defeef27c9ac7b85c97f030f93e4368bbc1.1711694852.git.baolin.wang@xxxxxxxxxxxxxxxxx/

On Tue, Apr 2, 2024 at 9:10 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> 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
>