linux-kernel - Re: [RFC PATCH 2/3] mm: handle large folio when large folio in VM

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Date:   Fri, 7 Jul 2023 23:56:14 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        ryan.roberts@....com, shy828301@...il.com,
        akpm@...ux-foundation.org, willy@...radead.org, david@...hat.com
Subject: Re: [RFC PATCH 2/3] mm: handle large folio when large folio in
 VM_LOCKED VMA range

On Fri, Jul 7, 2023 at 11:34 PM Yin, Fengwei <fengwei.yin@...el.com> wrote:
>
>
>
> On 7/8/2023 1:11 PM, Yu Zhao wrote:
> > On Fri, Jul 7, 2023 at 10:52 AM Yin Fengwei <fengwei.yin@...el.com> 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@...el.com>
> >> ---
> >>  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.
> Good point. We can't bail out here as return here means folio should
> not be reclaimed. My understanding is that we should skip the entries
> which is in the range of VM_LOCKED VMA. Will address this in coming
> version. Thanks.

Yes, that's what I mean. A wrapper would be cleaner:

    while () {
        ...
        if (vma->vm_flags & VM_LOCKED) {
            if (cant_mlock())
                goto next;
            ...
            return false;
        }
        ...
  next:
        pra->mapcount--;
    }