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

[Yu Zhao]

On Fri, Jul 7, 2023 at 10:52 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     |  3 ++-
>  2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 66117523d7d71..c7b8f0b008d81 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -637,7 +637,8 @@ 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);
>  }
>
> @@ -645,8 +646,12 @@ void munlock_folio(struct folio *folio);
>  static inline void munlock_vma_folio(struct folio *folio,
>                         struct vm_area_struct *vma, bool compound)
>  {
> -       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..7d6547d1bd096 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -817,7 +817,8 @@ static bool folio_referenced_one(struct folio *folio,
>                 address = pvmw.address;
>
>                 if ((vma->vm_flags & VM_LOCKED) &&
> -                   (!folio_test_large(folio) || !pvmw.pte)) {
> +                   (!folio_test_large(folio) || !pvmw.pte ||
> +                   folio_in_range(folio, vma, vma->vm_start, vma->vm_end))) {
>                         /* Restore the mlock which got missed */
>                         mlock_vma_folio(folio, vma, !pvmw.pte);
>                         page_vma_mapped_walk_done(&pvmw);

It needs to bail out if large but not within range so that the
references within the locked VMA can be ignored. Otherwise, a hot
locked portion can prevent a cold unlocked portion from getting
reclaimed.