Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

Nico Pache
2026-03-31T21:09:36Z

On Tue, Mar 31, 2026 at 3:04 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
>
> On 3/31/26 22:50, David Hildenbrand (Arm) wrote:
> > On 3/31/26 22:06, Lorenzo Stoakes (Oracle) wrote:
> >> On Tue, Mar 31, 2026 at 10:00:58PM +0200, David Hildenbrand (Arm) wrote:
> >>>
> >>> Right.
> >>>
> >>> The original code used
> >>>
> >>> bool mmap_locked = true;
> >>>
> >>> If the fix get squashed, maybe we should revert to that handling to
> >>> minimize the churn?
> >>
> >> Well collapse_single_pmd() and all descendents would need to be updated to
> >> invert the boolean, not sure it's worth it.
> >
> > The code is confusing me.
> >
> > In the old code, collapse_single_pmd() was called with "mmap_locked ==
> > true" and set "mmap_locked=false".
> >
> > Now we call it with "mmap_unlocked=false" and it sets "mmap_unlocked=true".
> >
> > So far so good.
> >
> > However, collapse_scan_pmd() consumed "mmap_locked", now it effectively
> > consumes "mmap_unlocked".
>
> Okay, I'm too confused for today and give up :)
>
> FWIW, this is the effective change, and the effective changes to
> mmap_unlocked and lock_dropped ... confuse me:

basically all the logic was inverted.

>
>
> mm/khugepaged.c | 146 +++++++++++++++++++++++++-----------------------
> 1 file changed, 76 insertions(+), 70 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 26c38934e829..3ffff9f98b2f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1253,7 +1253,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a
>
> static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> struct vm_area_struct *vma, unsigned long start_addr,
> - bool *mmap_locked, struct collapse_control *cc)
> + bool *lock_dropped, struct collapse_control *cc)
> {
> pmd_t *pmd;
> pte_t *pte, *_pte;
> @@ -1428,7 +1428,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> result = collapse_huge_page(mm, start_addr, referenced,
> unmapped, cc);
> /* collapse_huge_page will return with the mmap_lock released */
> - *mmap_locked = false;
> + *lock_dropped = true;
> }
> out:
> trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
> @@ -2420,6 +2420,67 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
> return result;
> }
>
> +/*
> + * Try to collapse a single PMD starting at a PMD aligned addr, and return
> + * the results.
> + */
> +static enum scan_result collapse_single_pmd(unsigned long addr,
> + struct vm_area_struct *vma, bool *lock_dropped,
> + struct collapse_control *cc)
> +{
> + struct mm_struct *mm = vma->vm_mm;
> + bool triggered_wb = false;
> + enum scan_result result;
> + struct file *file;
> + pgoff_t pgoff;
> +
> + mmap_assert_locked(mm);
> +
> + if (vma_is_anonymous(vma)) {
> + result = collapse_scan_pmd(mm, vma, addr, lock_dropped, cc);
> + goto end;
> + }
> +
> + file = get_file(vma->vm_file);
> + pgoff = linear_page_index(vma, addr);
> +
> + mmap_read_unlock(mm);
> + *lock_dropped = true;
> +retry:
> + result = collapse_scan_file(mm, addr, file, pgoff, cc);
> +
> + /*
> + * For MADV_COLLAPSE, when encountering dirty pages, try to writeback,
> + * then retry the collapse one time.
> + */
> + if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK &&
> + !triggered_wb && mapping_can_writeback(file->f_mapping)) {
> + const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT;
> + const loff_t lend = lstart + HPAGE_PMD_SIZE - 1;
> +
> + filemap_write_and_wait_range(file->f_mapping, lstart, lend);
> + triggered_wb = true;
> + goto retry;
> + }
> + fput(file);
> +
> + if (result == SCAN_PTE_MAPPED_HUGEPAGE) {
> + mmap_read_lock(mm);
> + if (collapse_test_exit_or_disable(mm))
> + result = SCAN_ANY_PROCESS;
> + else
> + result = try_collapse_pte_mapped_thp(mm, addr,
> + !cc->is_khugepaged);
> + if (result == SCAN_PMD_MAPPED)
> + result = SCAN_SUCCEED;
> + mmap_read_unlock(mm);
> + }
> +end:
> + if (cc->is_khugepaged && result == SCAN_SUCCEED)
> + ++khugepaged_pages_collapsed;
> + return result;
> +}
> +
> static void collapse_scan_mm_slot(unsigned int progress_max,
> enum scan_result *result, struct collapse_control *cc)
> __releases(&khugepaged_mm_lock)
> @@ -2481,46 +2542,21 @@ static void collapse_scan_mm_slot(unsigned int progress_max,
> VM_BUG_ON(khugepaged_scan.address & ~HPAGE_PMD_MASK);
>
> while (khugepaged_scan.address < hend) {
> - bool mmap_locked = true;
> + bool lock_dropped = false;
>
> cond_resched();
> if (unlikely(collapse_test_exit_or_disable(mm)))
> goto breakouterloop;
>
> - VM_BUG_ON(khugepaged_scan.address < hstart ||
> + VM_WARN_ON_ONCE(khugepaged_scan.address < hstart ||
> khugepaged_scan.address + HPAGE_PMD_SIZE >
> hend);
> - if (!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 = collapse_scan_file(mm,
> - khugepaged_scan.address, file, pgoff, cc);
> - fput(file);
> - if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> - mmap_read_lock(mm);
> - if (collapse_test_exit_or_disable(mm))
> - goto breakouterloop;
> - *result = try_collapse_pte_mapped_thp(mm,
> - khugepaged_scan.address, false);
> - if (*result == SCAN_PMD_MAPPED)
> - *result = SCAN_SUCCEED;
> - mmap_read_unlock(mm);
> - }
> - } else {
> - *result = collapse_scan_pmd(mm, vma,
> - khugepaged_scan.address, &mmap_locked, cc);
> - }
> -
> - if (*result == SCAN_SUCCEED)
> - ++khugepaged_pages_collapsed;
>
> + *result = collapse_single_pmd(khugepaged_scan.address,
> + vma, &lock_dropped, cc);
> /* move to next address */
> khugepaged_scan.address += HPAGE_PMD_SIZE;
> - if (!mmap_locked)
> + if (lock_dropped)
> /*
> * We released mmap_lock so break loop. Note
> * that we drop mmap_lock before all hugepage
> @@ -2795,7 +2831,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> unsigned long hstart, hend, addr;
> enum scan_result last_fail = SCAN_FAIL;
> int thps = 0;
> - bool mmap_locked = true;
> + bool mmap_unlocked = false;
>
> BUG_ON(vma->vm_start > start);
> BUG_ON(vma->vm_end < end);
> @@ -2817,13 +2853,12 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>
> for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> enum scan_result result = SCAN_FAIL;
> - bool triggered_wb = false;
>
> -retry:
> - if (!mmap_locked) {
> + if (mmap_unlocked) {
> cond_resched();
> mmap_read_lock(mm);
> - mmap_locked = true;
> + mmap_unlocked = false;
> + *lock_dropped = true;
> result = hugepage_vma_revalidate(mm, addr, false, &vma,
> cc);
> if (result != SCAN_SUCCEED) {
> @@ -2833,45 +2868,14 @@ 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, 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_unlocked, 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, 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_NON_PRESENT:
> @@ -2894,8 +2898,10 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>
> out_maybelock:
> /* Caller expects us to hold mmap_lock on return */
> - if (!mmap_locked)
> + if (mmap_unlocked) {
> + *lock_dropped = false;

Lorenzo, is this correct? At first glance, shouldn't this be
`locked_dropped = true`?

-- Nico

> mmap_read_lock(mm);
> + }
> out_nolock:
> mmap_assert_locked(mm);
> mmdrop(mm);
> --
> 2.43.0
>
>
> --
> Cheers,
>
> David
>