Re: [RFC PATCH 09/14] mm/khugepaged: record SCAN_PAGE_COMPOUND when scan_pmd() finds THP

[Yang Shi <shy828301@gmail.com>]

On Wed, Mar 9, 2022 at 4:46 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> On Wed, Mar 9, 2022 at 3:40 PM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Tue, Mar 8, 2022 at 1:35 PM Zach O'Keefe <zokeefe@google.com> wrote:
> > >
> > > When scanning an anon pmd to see if it's eligible for collapse, return
> > > SCAN_PAGE_COMPOUND if the pmd already maps a thp. This is consistent
> > > with handling when scanning file-backed memory.
> >
> > I'm not quite keen that we have to keep anon consistent with file for
> > this case. SCAN_PAGE_COMPOUND typically means the page is compound
> > page, but PTE mapped.
> >
>
> Good point.
>
> > And even SCAN_PMD_NULL is not returned every time when mm_find_pmd()
> > returns NULL. In addition, SCAN_PMD_NULL seems ambiguous to me. The
> > khugepaged actually sees non-present (migration) entry or trans huge
> > entry, so may rename it to SCAN_PMD_NOT_SUITABLE?
> >
>
> Sorry, I'm not sure I understand the suggestion here. What this patch
> would like to do, is to identify what pmds map thps. This will be
> important later, since if a user requests a collapse of an
> already-collapsed region, we want to return successfully (even if no
> work to be done).

Makes sense.

>
> Maybe there should be a SCAN_PMD_MAPPED used here instead? Just to not
> overload SCAN_PAGE_COMPOUND?

I see. SCAN_PMD_MAPPED sounds more self-explained and suitable IMHO.



>
> Though, note that when MADV_COLLAPSE supports file-backed memory, a
> similar check for pmd-mapping will need to be made on the file-side of
> things.
>
> > >
> > > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > > ---
> > >  mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++++++------
> > >  1 file changed, 35 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index ecbd3fc41c80..403578161a3b 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1011,6 +1011,38 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> > >                         vm_flags_ignore, vmap);
> > >  }
> > >
> > > +/*
> > > + * If returning NULL (meaning the pmd isn't mapped, isn't present, or thp),
> > > + * write the reason to *result.
> > > + */
> > > +static pmd_t *find_pmd_or_thp_or_none(struct mm_struct *mm,
> > > +                                     unsigned long address,
> > > +                                     int *result)
> > > +{
> > > +       pmd_t *pmd = mm_find_pmd_raw(mm, address);
> > > +       pmd_t pmde;
> > > +
> > > +       if (!pmd) {
> > > +               *result = SCAN_PMD_NULL;
> > > +               return NULL;
> > > +       }
> > > +
> > > +       pmde = pmd_read_atomic(pmd);
> > > +
> > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > +       /* See comments in pmd_none_or_trans_huge_or_clear_bad() */
> > > +       barrier();
> > > +#endif
> > > +       if (!pmd_present(pmde) || !pmd_none(pmde)) {
> > > +               *result = SCAN_PMD_NULL;
> > > +               return NULL;
> > > +       } else if (pmd_trans_huge(pmde)) {
> > > +               *result = SCAN_PAGE_COMPOUND;
> > > +               return NULL;
> > > +       }
> > > +       return pmd;
> > > +}
> > > +
> > >  /*
> > >   * Bring missing pages in from swap, to complete THP collapse.
> > >   * Only done if khugepaged_scan_pmd believes it is worthwhile.
> > > @@ -1212,9 +1244,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> > >                 goto out_nolock;
> > >         }
> > >
> > > -       pmd = mm_find_pmd(mm, address);
> > > +       pmd = find_pmd_or_thp_or_none(mm, address, &result);
> > >         if (!pmd) {
> > > -               result = SCAN_PMD_NULL;
> > >                 mmap_read_unlock(mm);
> > >                 goto out_nolock;
> > >         }
> > > @@ -1287,11 +1318,9 @@ static void scan_pmd(struct mm_struct *mm,
> > >         mmap_assert_locked(mm);
> > >         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> > >
> > > -       pmd = mm_find_pmd(mm, address);
> > > -       if (!pmd) {
> > > -               scan_result->result = SCAN_PMD_NULL;
> > > +       pmd = find_pmd_or_thp_or_none(mm, address, &scan_result->result);
> > > +       if (!pmd)
> > >                 goto out;
> > > -       }
> > >
> > >         memset(cc->node_load, 0, sizeof(cc->node_load));
> > >         pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> > > --
> > > 2.35.1.616.g0bdcbb4464-goog
> > >