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 12:44 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
>
> On 7/12/23 14:23, Yu Zhao wrote:
> > 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
> Yes. There is not difference here for pmd mapping of THPs and pte mappings of THPs
> if the only condition need check is whether the folio is within VMA range or not.
>
> But let me add Huge for confirmation.
>
>
> > 2. make the if
> >         if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED) &&
> >             folio_within_vma())
> >                 mlock_folio(folio);
> !folio_test_large(folio) was kept here by purpose. For normal 4K page, don't need
> to call folio_within_vma() which is heavy for normal 4K page.

I suspected you would think so -- I don't think it would make any
measurable (for systems with mostly large folios, it would actually be
an extra work). Since we have many places like this once, probably we
could wrap folio_test_large() into folio_within_vma() and call it
large_folio_within_vma(), if you feel it's necessary.

> >> @@ -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;
> I tried not to use goto as much as possible. I suppose you mean:
>
>     if (!should_restore_lock())
>         goto dec_pra_mapcount; (I may use continue here. :)).

should_restore_lock() is just folio_within_vma() -- see the comment
above. "continue" looks good to me too (prefer not to add more indents
to the functions below).

>     mlock_vma_folio();
>     page_vma_mapped_walk_done()
>    ...
>
> Right?

Right.