Re: [PATCH v5 2/6] mm: remap unused subpages to shared zeropage when splitting isolated thp

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On Wed, Oct 23, 2024 at 10:51 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote:
>
> On 23/10/2024 17:21, Zi Yan wrote:
> > On 30 Aug 2024, at 6:03, Usama Arif wrote:
> >
> >> From: Yu Zhao <yuzhao@xxxxxxxxxx>
> >>
> >> Here being unused means containing only zeros and inaccessible to
> >> userspace. When splitting an isolated thp under reclaim or migration,
> >> the unused subpages can be mapped to the shared zeropage, hence saving
> >> memory. This is particularly helpful when the internal
> >> fragmentation of a thp is high, i.e. it has many untouched subpages.
> >>
> >> This is also a prerequisite for THP low utilization shrinker which will
> >> be introduced in later patches, where underutilized THPs are split, and
> >> the zero-filled pages are freed saving memory.
> >>
> >> Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
> >> Tested-by: Shuang Zhai <zhais@xxxxxxxxxx>
> >> Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
> >> ---
> >>  include/linux/rmap.h |  7 ++++-
> >>  mm/huge_memory.c     |  8 ++---
> >>  mm/migrate.c         | 72 ++++++++++++++++++++++++++++++++++++++------
> >>  mm/migrate_device.c  |  4 +--
> >>  4 files changed, 75 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> >> index 91b5935e8485..d5e93e44322e 100644
> >> --- a/include/linux/rmap.h
> >> +++ b/include/linux/rmap.h
> >> @@ -745,7 +745,12 @@ int folio_mkclean(struct folio *);
> >>  int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,
> >>                    struct vm_area_struct *vma);
> >>
> >> -void remove_migration_ptes(struct folio *src, struct folio *dst, bool locked);
> >> +enum rmp_flags {
> >> +    RMP_LOCKED              = 1 << 0,
> >> +    RMP_USE_SHARED_ZEROPAGE = 1 << 1,
> >> +};
> >> +
> >> +void remove_migration_ptes(struct folio *src, struct folio *dst, int flags);
> >>
> >>  /*
> >>   * rmap_walk_control: To control rmap traversing for specific needs
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 0c48806ccb9a..af60684e7c70 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -3020,7 +3020,7 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> >>      return false;
> >>  }
> >>
> >> -static void remap_page(struct folio *folio, unsigned long nr)
> >> +static void remap_page(struct folio *folio, unsigned long nr, int flags)
> >>  {
> >>      int i = 0;
> >>
> >> @@ -3028,7 +3028,7 @@ static void remap_page(struct folio *folio, unsigned long nr)
> >>      if (!folio_test_anon(folio))
> >>              return;
> >>      for (;;) {
> >> -            remove_migration_ptes(folio, folio, true);
> >> +            remove_migration_ptes(folio, folio, RMP_LOCKED | flags);
> >>              i += folio_nr_pages(folio);
> >>              if (i >= nr)
> >>                      break;
> >> @@ -3240,7 +3240,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> >>
> >>      if (nr_dropped)
> >>              shmem_uncharge(folio->mapping->host, nr_dropped);
> >> -    remap_page(folio, nr);
> >> +    remap_page(folio, nr, PageAnon(head) ? RMP_USE_SHARED_ZEROPAGE : 0);
> >>
> >>      /*
> >>       * set page to its compound_head when split to non order-0 pages, so
> >> @@ -3542,7 +3542,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >>              if (mapping)
> >>                      xas_unlock(&xas);
> >>              local_irq_enable();
> >> -            remap_page(folio, folio_nr_pages(folio));
> >> +            remap_page(folio, folio_nr_pages(folio), 0);
> >>              ret = -EAGAIN;
> >>      }
> >>
> >> diff --git a/mm/migrate.c b/mm/migrate.c
> >> index 6f9c62c746be..d039863e014b 100644
> >> --- a/mm/migrate.c
> >> +++ b/mm/migrate.c
> >> @@ -204,13 +204,57 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
> >>      return true;
> >>  }
> >>
> >> +static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> >> +                                      struct folio *folio,
> >> +                                      unsigned long idx)
> >> +{
> >> +    struct page *page = folio_page(folio, idx);
> >> +    bool contains_data;
> >> +    pte_t newpte;
> >> +    void *addr;
> >> +
> >> +    VM_BUG_ON_PAGE(PageCompound(page), page);
> >
> > This should be:
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index e950fd62607f..7ffdbe078aa7 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -206,7 +206,8 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
> >         pte_t newpte;
> >         void *addr;
> >
> > -       VM_BUG_ON_PAGE(PageCompound(page), page);
> > +       if (PageCompound(page))
> > +               return false;
> >         VM_BUG_ON_PAGE(!PageAnon(page), page);
> >         VM_BUG_ON_PAGE(!PageLocked(page), page);
> >         VM_BUG_ON_PAGE(pte_present(*pvmw->pte), page);
> >
> > Otherwise, splitting anonymous large folios to non order-0 ones just
> > triggers this BUG_ON.
> >
>
> That makes sense, would you like to send the fix?
>
> Adding Yu Zhao to "To" incase he has any objections.

LGTM. Thanks!





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