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

Baolin Wang
2025-04-23T06:49:34Z

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?

> ---
> 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;

> + 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?

>
> /* 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 */