Re: [PATCH mm-unstable v2 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Nico Pache]

On Thu, Feb 26, 2026 at 1:20 PM Nico Pache <npache@xxxxxxxxxx> wrote:
>
> On Thu, Feb 26, 2026 at 2:24 AM Baolin Wang
> <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
> >
> >
> >
> > On 2/26/26 9:29 AM, Nico Pache wrote:
> > > The khugepaged daemon and madvise_collapse have two different
> > > implementations that do almost the same thing.
> > >
> > > Create collapse_single_pmd to increase code reuse and create an entry
> > > point to these two users.
> > >
> > > Refactor madvise_collapse and collapse_scan_mm_slot to use the new
> > > collapse_single_pmd function. This introduces a minor behavioral change
> > > that is most likely an undiscovered bug. The current implementation of
> > > khugepaged tests collapse_test_exit_or_disable before calling
> > > collapse_pte_mapped_thp, but we weren't doing it in the madvise_collapse
> > > case. By unifying these two callers madvise_collapse now also performs
> > > this check. We also modify the return value to be SCAN_ANY_PROCESS which
> > > properly indicates that this process is no longer valid to operate on.
> > >
> > > We also guard the khugepaged_pages_collapsed variable to ensure its only
> > > incremented for khugepaged.
> > >
> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > > Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> > > ---
> >
> > [snip]
> >
> > >       for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> > >               enum scan_result result = SCAN_FAIL;
> > > -             bool triggered_wb = false;
> > >
> > > -retry:
> > >               if (!mmap_locked) {
> > >                       cond_resched();
> > >                       mmap_read_lock(mm);
> > >                       mmap_locked = true;
> > > +                     *lock_dropped = true;
> > IIUC, this should be '*lock_dropped = false', right?
>
> Yes! Thanks for catching that :) As David and others have pointed out,
> this lock handling here might be unnecessary and better placed in
> collapse_single_pmd(). I meant to look into that before posting this
> but it slipped my mind.

On second pass, no, I think we should drop this line altogether. If
(!mmap_locked) -> we have either just completed a collapse, or we
tried file collapse on a 2MB region. Collapse_single_pmd would report
this, and we would have already set lock_dropped.

>
> >
> > >                       result = hugepage_vma_revalidate(mm, addr, false, &vma,
> > >                                                        cc);
> > >                       if (result  != SCAN_SUCCEED) {
> > > @@ -2836,46 +2872,20 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> > >                       hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
> > >               }
> > >               mmap_assert_locked(mm);
> > > -             if (!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;
> > > -                     *lock_dropped = true;
> > > -                     result = collapse_scan_file(mm, addr, file, pgoff, NULL, cc);
> > >
> > > -                     if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb &&
> > > -                         mapping_can_writeback(file->f_mapping)) {
> > > -                             loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> > > -                             loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> > > +             result = collapse_single_pmd(addr, vma, &mmap_locked, NULL, cc);
> > >
> > > -                             filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> > > -                             triggered_wb = true;
> > > -                             fput(file);
> > > -                             goto retry;
> > > -                     }
> > > -                     fput(file);
> > > -             } else {
> > > -                     result = collapse_scan_pmd(mm, vma, addr, &mmap_locked, NULL, cc);
> > > -             }
> > >               if (!mmap_locked)
> > >                       *lock_dropped = true;
> > >
> > > -handle_result:
> > >               switch (result) {
> > >               case SCAN_SUCCEED:
> > >               case SCAN_PMD_MAPPED:
> > >                       ++thps;
> > >                       break;
> > > -             case SCAN_PTE_MAPPED_HUGEPAGE:
> > > -                     BUG_ON(mmap_locked);
> > > -                     mmap_read_lock(mm);
> > > -                     result = try_collapse_pte_mapped_thp(mm, addr, true);
> > > -                     mmap_read_unlock(mm);
> > > -                     goto handle_result;
> > >               /* Whitelisted set of results where continuing OK */
> > >               case SCAN_NO_PTE_TABLE:
> > > +             case SCAN_PTE_MAPPED_HUGEPAGE:
> > >               case SCAN_PTE_NON_PRESENT:
> > >               case SCAN_PTE_UFFD_WP:
> > >               case SCAN_LACK_REFERENCED_PAGE:
> >