Re: [RFC PATCH v2 2/3] mm: handle large folio when large folio in VM_LOCKED VMA range

[Yu Zhao]

On Wed, Jul 12, 2023 at 12:02 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
> If large folio is in the range of VM_LOCKED VMA, it should be
> mlocked to avoid being picked by page reclaim. Which may split
> the large folio and then mlock each pages again.
>
> Mlock this kind of large folio to prevent them being picked by
> page reclaim.
>
> For the large folio which cross the boundary of VM_LOCKED VMA,
> we'd better not to mlock it. So if the system is under memory
> pressure, this kind of large folio will be split and the pages
> ouf of VM_LOCKED VMA can be reclaimed.
>
> Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> ---
>  mm/internal.h | 11 ++++++++---
>  mm/rmap.c     | 34 +++++++++++++++++++++++++++-------
>  2 files changed, 35 insertions(+), 10 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index c7dd15d8de3ef..776141de2797a 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -643,7 +643,8 @@ static inline void mlock_vma_folio(struct folio *folio,
>          *    still be set while VM_SPECIAL bits are added: so ignore it then.
>          */
>         if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED) &&
> -           (compound || !folio_test_large(folio)))
> +           (compound || !folio_test_large(folio) ||
> +           folio_in_range(folio, vma, vma->vm_start, vma->vm_end)))
>                 mlock_folio(folio);
>  }

This can be simplified:
1. remove the compound parameter
2. make the if
        if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED) &&
            folio_within_vma())
                mlock_folio(folio);

> @@ -651,8 +652,12 @@ void munlock_folio(struct folio *folio);
>  static inline void munlock_vma_folio(struct folio *folio,
>                         struct vm_area_struct *vma, bool compound)

Remove the compound parameter here too.

>  {
> -       if (unlikely(vma->vm_flags & VM_LOCKED) &&
> -           (compound || !folio_test_large(folio)))
> +       /*
> +        * To handle the case that a mlocked large folio is unmapped from VMA
> +        * piece by piece, allow munlock the large folio which is partially
> +        * mapped to VMA.
> +        */
> +       if (unlikely(vma->vm_flags & VM_LOCKED))
>                 munlock_folio(folio);
>  }
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2668f5ea35342..455f415d8d9ca 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -803,6 +803,14 @@ struct folio_referenced_arg {
>         unsigned long vm_flags;
>         struct mem_cgroup *memcg;
>  };
> +
> +static inline bool should_restore_mlock(struct folio *folio,
> +               struct vm_area_struct *vma, bool pmd_mapped)
> +{
> +       return !folio_test_large(folio) ||
> +                       pmd_mapped || folio_within_vma(folio, vma);
> +}

This is just folio_within_vma() :)

>  /*
>   * arg: folio_referenced_arg will be passed
>   */
> @@ -816,13 +824,25 @@ static bool folio_referenced_one(struct folio *folio,
>         while (page_vma_mapped_walk(&pvmw)) {
>                 address = pvmw.address;
>
> -               if ((vma->vm_flags & VM_LOCKED) &&
> -                   (!folio_test_large(folio) || !pvmw.pte)) {
> -                       /* Restore the mlock which got missed */
> -                       mlock_vma_folio(folio, vma, !pvmw.pte);
> -                       page_vma_mapped_walk_done(&pvmw);
> -                       pra->vm_flags |= VM_LOCKED;
> -                       return false; /* To break the loop */
> +               if (vma->vm_flags & VM_LOCKED) {
> +                       if (should_restore_mlock(folio, vma, !pvmw.pte)) {
> +                               /* Restore the mlock which got missed */
> +                               mlock_vma_folio(folio, vma, !pvmw.pte);
> +                               page_vma_mapped_walk_done(&pvmw);
> +                               pra->vm_flags |= VM_LOCKED;
> +                               return false; /* To break the loop */
> +                       } else {

There is no need for "else", or just

  if (!folio_within_vma())
    goto dec_pra_mapcount;

> +                               /*
> +                                * For large folio cross VMA boundaries, it's
> +                                * expected to be picked  by page reclaim. But
> +                                * should skip reference of pages which are in
> +                                * the range of VM_LOCKED vma. As page reclaim
> +                                * should just count the reference of pages out
> +                                * the range of VM_LOCKED vma.
> +                                */
> +                               pra->mapcount--;
> +                               continue;
> +                       }
>                 }