Re: [PATCH v4 01/12] introduce khugepaged_collapse_single_pmd to unify khugepaged and madvise_collapse

Nico Pache
2025-04-23T07:06:31Z

On Wed, Apr 23, 2025 at 12:44 AM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2025/4/17 08:02, 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>
>
> Can you add a prefix 'khugepaged:' for the subject line?
I had that originally but the subject line is already extremely long.
>
> > ---
> > mm/khugepaged.c | 92 ++++++++++++++++++++++++-------------------------
> > 1 file changed, 46 insertions(+), 46 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index b8838ba8207a..cecadc4239e7 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2363,6 +2363,48 @@ 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 vm_area_struct *vma, bool *mmap_locked,
> > + struct collapse_control *cc)
> > +{
> > + int result = SCAN_FAIL;
> > + struct mm_struct *mm = vma->vm_mm;
> > + unsigned long tva_flags = cc->is_khugepaged ? TVA_ENFORCE_SYSFS : 0;
> > +
> > + if (thp_vma_allowable_order(vma, vma->vm_flags,
> > + tva_flags, PMD_ORDER)) {
>
> We've already checked the thp_vma_allowable_order() before calling this
> function, why check again?
>
> > + 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);
> > + 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);
>
> why drop the following check?
> if (*result == SCAN_PMD_MAPPED)
> *result = SCAN_SUCCEED;

Good catch! When generalizing this for madvise_collapse i forgot to
properly handle the khugepaged case of PMD_MAPPED==SUCCEED.
>
> > + mmap_read_unlock(mm);
> > + }
> > + } else {
> > + result = hpage_collapse_scan_pmd(mm, vma, addr,
> > + mmap_locked, cc);
> > + }
> > + if (cc->is_khugepaged && result == SCAN_SUCCEED)
> > + ++khugepaged_pages_collapsed;
> > + }
> > +end:
> > + return result;
> > +}
> > +
> > static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> > struct collapse_control *cc)
> > __releases(&khugepaged_mm_lock)
> > @@ -2437,33 +2479,9 @@ 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,
> > + vma, &mmap_locked, cc);
>
> If the khugepaged_collapse_single_pmd() returns a failure caused by
> hpage_collapse_test_exit_or_disable(), we should break out of the loop
> according to the original logic. But you've changed the action in this
> patch, is this intentional?

Nope, not intentional! Thanks for pointing that out. I'll get that fixed!

Thanks for the in depth review ! I'll work on cleaning up these corner cases.
>
> >
> > /* move to next address */
> > khugepaged_scan.address += HPAGE_PMD_SIZE;
> > @@ -2783,36 +2801,18 @@ 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, 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:
>