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

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

On Sun, Apr 13, 2025 at 12:31 AM Zi Yan <ziy@nvidia.com> wrote:
>
> On 12 Apr 2025, at 4:58, Barry Song wrote:
>
> > From: Barry Song <v-songbaohua@oppo.com>
> >
> > 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:
                                                   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@linux.alibaba.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: Ryan Roberts <ryan.roberts@arm.com>
> > Cc: Zi Yan <ziy@nvidia.com>
> > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > ---
> >  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 &&
> > +                         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