Re: [RFC v2 1/9] introduce khugepaged_collapse_single_pmd to unify khugepaged and madvise_collapse

[Nico Pache <npache@redhat.com>]

On Tue, Feb 18, 2025 at 9:26 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 11/02/2025 00:30, Nico Pache wrote:
> > The khugepaged daemon and madvise_collapse have two different
> > implementations that do almost the same thing.
> >
> > Create khugepaged_collapse_single_pmd to increase code
> > reuse and create an entry point for future khugepaged changes.
> >
> > Refactor madvise_collapse and khugepaged_scan_mm_slot to use
> > the new khugepaged_collapse_single_pmd function.
> >
> > Signed-off-by: Nico Pache <npache@redhat.com>
> > ---
> >  mm/khugepaged.c | 96 +++++++++++++++++++++++++------------------------
> >  1 file changed, 50 insertions(+), 46 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 5f0be134141e..46faee67378b 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2365,6 +2365,52 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
> >  }
> >  #endif
> >
> > +/*
> > + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> > + * the results.
> > + */
> > +static int khugepaged_collapse_single_pmd(unsigned long addr, struct mm_struct *mm,
> > +                                struct vm_area_struct *vma, bool *mmap_locked,
> > +                                struct collapse_control *cc)
>
> nit: given the vma links to the mm is it really neccessary to pass both? Why not
> just pass vma?
Ah good point!
>
> > +{
> > +     int result = SCAN_FAIL;
> > +     unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
> > +
> > +     if (!*mmap_locked) {
> > +             mmap_read_lock(mm);
> > +             *mmap_locked = true;
> > +     }
>
> AFAICT, the read lock is always held when khugepaged_collapse_single_pmd() is
> called. Perhaps VM_WARN_ON(!*mmap_locked) would be more appropriate?

Hmm, I'm actually not sure why that's there. Its probably left over
from a previous change I was trying a while back.

>
> > +
> > +     if (thp_vma_allowable_order(vma, vma->vm_flags,
> > +                                     tva_flags, PMD_ORDER)) {
> > +             if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
>
> I guess it was like this before, but what's the relevance of CONFIG_SHMEM?
> Surely this should work for any file if CONFIG_READ_ONLY_THP_FOR_FS is enabled?

Yeah I think David brought up a similar point during one of our
meetings. I'm just combining the collapse users, so I'll leave it for
now.

>
> > +                     struct file *file = get_file(vma->vm_file);
> > +                     pgoff_t pgoff = linear_page_index(vma, addr);
> > +
> > +                     mmap_read_unlock(mm);
> > +                     *mmap_locked = false;
> > +                     result = hpage_collapse_scan_file(mm, addr, file, pgoff,
> > +                                                       cc);
> > +                     fput(file);
> > +                     if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > +                             mmap_read_lock(mm);
> > +                             if (hpage_collapse_test_exit_or_disable(mm))
> > +                                     goto end;
> > +                             result = collapse_pte_mapped_thp(mm, addr,
> > +                                                              !cc->is_khugepaged);
> > +                             mmap_read_unlock(mm);
> > +                     }
> > +             } else {
> > +                     result = hpage_collapse_scan_pmd(mm, vma, addr,
> > +                                                      mmap_locked, cc);
> > +             }
> > +             if (result == SCAN_SUCCEED || result == SCAN_PMD_MAPPED)
> > +                     ++khugepaged_pages_collapsed;
>
> Looks like this counter was previously only incremented for the scan path, not
> for the madvise_collapse path. Not sure if that's a problem?

Yep! Usama noted that too, already fixed in the next version :)

Thanks!
-- Nico

>
> Thanks,
> Ryan
>
> > +     }
> > +end:
> > +     return result;
> > +}
> > +
> >  static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >                                           struct collapse_control *cc)
> >       __releases(&khugepaged_mm_lock)
> > @@ -2439,33 +2485,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >                       VM_BUG_ON(khugepaged_scan.address < hstart ||
> >                                 khugepaged_scan.address + HPAGE_PMD_SIZE >
> >                                 hend);
> > -                     if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
> > -                             struct file *file = get_file(vma->vm_file);
> > -                             pgoff_t pgoff = linear_page_index(vma,
> > -                                             khugepaged_scan.address);
> >
> > -                             mmap_read_unlock(mm);
> > -                             mmap_locked = false;
> > -                             *result = hpage_collapse_scan_file(mm,
> > -                                     khugepaged_scan.address, file, pgoff, cc);
> > -                             fput(file);
> > -                             if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> > -                                     mmap_read_lock(mm);
> > -                                     if (hpage_collapse_test_exit_or_disable(mm))
> > -                                             goto breakouterloop;
> > -                                     *result = collapse_pte_mapped_thp(mm,
> > -                                             khugepaged_scan.address, false);
> > -                                     if (*result == SCAN_PMD_MAPPED)
> > -                                             *result = SCAN_SUCCEED;
> > -                                     mmap_read_unlock(mm);
> > -                             }
> > -                     } else {
> > -                             *result = hpage_collapse_scan_pmd(mm, vma,
> > -                                     khugepaged_scan.address, &mmap_locked, cc);
> > -                     }
> > -
> > -                     if (*result == SCAN_SUCCEED)
> > -                             ++khugepaged_pages_collapsed;
> > +                     *result = khugepaged_collapse_single_pmd(khugepaged_scan.address,
> > +                                             mm, vma, &mmap_locked, cc);
> >
> >                       /* move to next address */
> >                       khugepaged_scan.address += HPAGE_PMD_SIZE;
> > @@ -2785,36 +2807,18 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
> >               mmap_assert_locked(mm);
> >               memset(cc->node_load, 0, sizeof(cc->node_load));
> >               nodes_clear(cc->alloc_nmask);
> > -             if (IS_ENABLED(CONFIG_SHMEM) && !vma_is_anonymous(vma)) {
> > -                     struct file *file = get_file(vma->vm_file);
> > -                     pgoff_t pgoff = linear_page_index(vma, addr);
> >
> > -                     mmap_read_unlock(mm);
> > -                     mmap_locked = false;
> > -                     result = hpage_collapse_scan_file(mm, addr, file, pgoff,
> > -                                                       cc);
> > -                     fput(file);
> > -             } else {
> > -                     result = hpage_collapse_scan_pmd(mm, vma, addr,
> > -                                                      &mmap_locked, cc);
> > -             }
> > +             result = khugepaged_collapse_single_pmd(addr, mm, vma, &mmap_locked, cc);
> > +
> >               if (!mmap_locked)
> >                       *prev = NULL;  /* Tell caller we dropped mmap_lock */
> >
> > -handle_result:
> >               switch (result) {
> >               case SCAN_SUCCEED:
> >               case SCAN_PMD_MAPPED:
> >                       ++thps;
> >                       break;
> >               case SCAN_PTE_MAPPED_HUGEPAGE:
> > -                     BUG_ON(mmap_locked);
> > -                     BUG_ON(*prev);
> > -                     mmap_read_lock(mm);
> > -                     result = collapse_pte_mapped_thp(mm, addr, true);
> > -                     mmap_read_unlock(mm);
> > -                     goto handle_result;
> > -             /* Whitelisted set of results where continuing OK */
> >               case SCAN_PMD_NULL:
> >               case SCAN_PTE_NON_PRESENT:
> >               case SCAN_PTE_UFFD_WP:
>