Re: [RFC PATCH] mm: don't promote exclusive file folios of dying processes

[Baolin Wang]

On 2025/4/16 15:48, Barry Song wrote:
> On Sun, Apr 13, 2025 at 12:31 AM Zi Yan <ziy@xxxxxxxxxx> wrote:
> > On 12 Apr 2025, at 4:58, Barry Song wrote:
> >
> > > From: Barry Song <v-songbaohua@xxxxxxxx>
> > >
> > > Promoting exclusive file folios of a dying process is unnecessary and
> > > harmful. For example, while Firefox is killed and LibreOffice is
> > > launched, activating Firefox's young file-backed folios makes it
> > > harder to reclaim memory that LibreOffice doesn't use at all.
> > >
> > > An exiting process is unlikely to be restarted right away—it's
> > > either terminated by the user or killed by the OOM handler.
> >
> > The proposal looks reasonable to me. Do you have any performance number
> > about the improvement?
>
> Tangquan ran the test on Android phones and saw 3% improvement on
> refault/thrashing things:

Good.

>                                                     w/o patch           w/patch
> workingset_refault_anon    2215933          2146602            3.13%
> workingset_refault_file       9859208          9646518             2.16%
> pswpin                                2411086          2337790             3.04%
> pswpout                              6482838          6264865             3.36%
>
> A further demotion of exclusive file folios can improvement more, but
> might be controversial. it could be a separate patch later.
>
> > > Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > Cc: Oscar Salvador <osalvador@xxxxxxx>
> > > Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > > Cc: Zi Yan <ziy@xxxxxxxxxx>
> > > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > > ---
> > >   mm/huge_memory.c |  4 ++--
> > >   mm/internal.h    | 19 +++++++++++++++++++
> > >   mm/memory.c      |  9 ++++++++-
> > >   3 files changed, 29 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index e97a97586478..05b83d2fcbb6 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -2264,8 +2264,8 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > >                         * Use flush_needed to indicate whether the PMD entry
> > >                         * is present, instead of checking pmd_present() again.
> > >                         */
> > > -                     if (flush_needed && pmd_young(orig_pmd) &&
> > > -                         likely(vma_has_recency(vma)))
> > > +                     if (!exclusive_folio_of_dying_process(folio, vma) && flush_needed &&

Nit: I prefer to check 'flush_needed' first to make sure it is a present 
pte. Otherwise look good to me.

> > > +                         pmd_young(orig_pmd) && likely(vma_has_recency(vma)))
> > >                                folio_mark_accessed(folio);
> > >                }
> > >
> > > diff --git a/mm/internal.h b/mm/internal.h
> > > index 4e0ea83aaf1c..666de96a293d 100644
> > > --- a/mm/internal.h
> > > +++ b/mm/internal.h
> > > @@ -11,6 +11,7 @@
> > >   #include <linux/khugepaged.h>
> > >   #include <linux/mm.h>
> > >   #include <linux/mm_inline.h>
> > > +#include <linux/oom.h>
> > >   #include <linux/pagemap.h>
> > >   #include <linux/pagewalk.h>
> > >   #include <linux/rmap.h>
> > > @@ -130,6 +131,24 @@ static inline int folio_nr_pages_mapped(const struct folio *folio)
> > >        return atomic_read(&folio->_nr_pages_mapped) & FOLIO_PAGES_MAPPED;
> > >   }
> > >
> > > +/*
> > > + * Return true if a folio is exclusive and belongs to an exiting or
> > > + * oom-reaped process; otherwise, return false.
> > > + */
> > > +static inline bool exclusive_folio_of_dying_process(struct folio *folio,
> > > +             struct vm_area_struct *vma)
> > > +{
> > > +     if (folio_maybe_mapped_shared(folio))
> > > +             return false;
> > > +
> > > +     if (!atomic_read(&vma->vm_mm->mm_users))
> > > +             return true;
> > > +     if (check_stable_address_space(vma->vm_mm))
> > > +             return true;
> > > +
> > > +     return false;
> > > +}
> > > +
> > >   /*
> > >    * Retrieve the first entry of a folio based on a provided entry within the
> > >    * folio. We cannot rely on folio->swap as there is no guarantee that it has
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index b9e8443aaa86..cab69275e473 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -1515,7 +1515,14 @@ static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
> > >                                *force_flush = true;
> > >                        }
> > >                }
> > > -             if (pte_young(ptent) && likely(vma_has_recency(vma)))
> > > +
> > > +             /*
> > > +              * Skip marking exclusive file folios as accessed for processes that are
> > > +              * exiting or have been reaped due to OOM. This prevents unnecessary
> > > +              * promotion of folios that won't benefit the new process being launched.
> > > +              */
> > > +             if (!exclusive_folio_of_dying_process(folio, vma) && pte_young(ptent) &&
> > > +                             likely(vma_has_recency(vma)))
> > >                        folio_mark_accessed(folio);
> > >                rss[mm_counter(folio)] -= nr;
> > >        } else {
> > > --
> > > 2.39.3 (Apple Git-146)
> >
> >
> > --
> > Best Regards,
> > Yan, Zi
>
> Thanks
> Barry