On Wed, Jul 16, 2025 at 7:53 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 14.07.25 02:31, Nico Pache wrote:
> > generalize the order of the __collapse_huge_page_* functions
> > to support future mTHP collapse.
> >
> > mTHP collapse can suffer from incosistant behavior, and memory waste
> > "creep". disable swapin and shared support for mTHP collapse.
> >
> > No functional changes in this patch.
> >
> > Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> > Co-developed-by: Dev Jain <dev.jain@xxxxxxx>
> > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> > ---
> > mm/khugepaged.c | 49 +++++++++++++++++++++++++++++++------------------
> > 1 file changed, 31 insertions(+), 18 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index cc9a35185604..ee54e3c1db4e 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -552,15 +552,17 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > unsigned long address,
> > pte_t *pte,
> > struct collapse_control *cc,
> > - struct list_head *compound_pagelist)
> > + struct list_head *compound_pagelist,
> > + u8 order)
>
> u8 ... (applies to all instances)
Fixed all instances of this (other than those that need to stay)
>
> > {
> > struct page *page = NULL;
> > struct folio *folio = NULL;
> > pte_t *_pte;
> > int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> > bool writable = false;
> > + int scaled_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
>
> "scaled_max_ptes_none" maybe?
done!
>
> >
> > - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > + for (_pte = pte; _pte < pte + (1 << order);
> > _pte++, address += PAGE_SIZE) {
> > pte_t pteval = ptep_get(_pte);
> > if (pte_none(pteval) || (pte_present(pteval) &&
> > @@ -568,7 +570,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > ++none_or_zero;
> > if (!userfaultfd_armed(vma) &&
> > (!cc->is_khugepaged ||
> > - none_or_zero <= khugepaged_max_ptes_none)) {
> > + none_or_zero <= scaled_none)) {
> > continue;
> > } else {
> > result = SCAN_EXCEED_NONE_PTE;
> > @@ -596,8 +598,8 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > /* See hpage_collapse_scan_pmd(). */
> > if (folio_maybe_mapped_shared(folio)) {
> > ++shared;
> > - if (cc->is_khugepaged &&
> > - shared > khugepaged_max_ptes_shared) {
> > + if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
> > + shared > khugepaged_max_ptes_shared)) {
>
> Please add a comment why we do something different with PMD. As
> commenting below, does this deserve a TODO?
>
> > result = SCAN_EXCEED_SHARED_PTE;
> > count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> > goto out;
> > @@ -698,13 +700,14 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> > struct vm_area_struct *vma,
> > unsigned long address,
> > spinlock_t *ptl,
> > - struct list_head *compound_pagelist)
> > + struct list_head *compound_pagelist,
> > + u8 order)
> > {
> > struct folio *src, *tmp;
> > pte_t *_pte;
> > pte_t pteval;
> >
> > - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> > + for (_pte = pte; _pte < pte + (1 << order);
> > _pte++, address += PAGE_SIZE) {
> > pteval = ptep_get(_pte);
> > if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> > @@ -751,7 +754,8 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> > pmd_t *pmd,
> > pmd_t orig_pmd,
> > struct vm_area_struct *vma,
> > - struct list_head *compound_pagelist)
> > + struct list_head *compound_pagelist,
> > + u8 order)
> > {
> > spinlock_t *pmd_ptl;
> >
> > @@ -768,7 +772,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> > * Release both raw and compound pages isolated
> > * in __collapse_huge_page_isolate.
> > */
> > - release_pte_pages(pte, pte + HPAGE_PMD_NR, compound_pagelist);
> > + release_pte_pages(pte, pte + (1 << order), compound_pagelist);
> > }
> >
> > /*
> > @@ -789,7 +793,7 @@ static void __collapse_huge_page_copy_failed(pte_t *pte,
> > static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> > pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> > unsigned long address, spinlock_t *ptl,
> > - struct list_head *compound_pagelist)
> > + struct list_head *compound_pagelist, u8 order)
> > {
> > unsigned int i;
> > int result = SCAN_SUCCEED;
> > @@ -797,7 +801,7 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> > /*
> > * Copying pages' contents is subject to memory poison at any iteration.
> > */
> > - for (i = 0; i < HPAGE_PMD_NR; i++) {
> > + for (i = 0; i < (1 << order); i++) {
> > pte_t pteval = ptep_get(pte + i);
> > struct page *page = folio_page(folio, i);
> > unsigned long src_addr = address + i * PAGE_SIZE;
> > @@ -816,10 +820,10 @@ static int __collapse_huge_page_copy(pte_t *pte, struct folio *folio,
> >
> > if (likely(result == SCAN_SUCCEED))
> > __collapse_huge_page_copy_succeeded(pte, vma, address, ptl,
> > - compound_pagelist);
> > + compound_pagelist, order);
> > else
> > __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> > - compound_pagelist);
> > + compound_pagelist, order);
> >
> > return result;
> > }
> > @@ -994,11 +998,11 @@ static int check_pmd_still_valid(struct mm_struct *mm,
> > static int __collapse_huge_page_swapin(struct mm_struct *mm,
> > struct vm_area_struct *vma,
> > unsigned long haddr, pmd_t *pmd,
> > - int referenced)
> > + int referenced, u8 order)
> > {
> > int swapped_in = 0;
> > vm_fault_t ret = 0;
> > - unsigned long address, end = haddr + (HPAGE_PMD_NR * PAGE_SIZE);
> > + unsigned long address, end = haddr + (PAGE_SIZE << order);
> > int result;
> > pte_t *pte = NULL;
> > spinlock_t *ptl;
> > @@ -1029,6 +1033,15 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> > if (!is_swap_pte(vmf.orig_pte))
> > continue;
> >
> > + /* Dont swapin for mTHP collapse */
>
> Should we turn this into a TODO, because it's something to figure out
> regarding the scaling etc?
Good idea, I changed both of these into TODOs
>
> > + if (order != HPAGE_PMD_ORDER) {
> > + count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
> > + pte_unmap(pte);
> > + mmap_read_unlock(mm);
> > + result = SCAN_EXCEED_SWAP_PTE;
> > + goto out;
> > + }
> > +
> > vmf.pte = pte;
> > vmf.ptl = ptl;
> > ret = do_swap_page(&vmf);
> > @@ -1149,7 +1162,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > * that case. Continuing to collapse causes inconsistency.
> > */
> > result = __collapse_huge_page_swapin(mm, vma, address, pmd,
> > - referenced);
> > + referenced, HPAGE_PMD_ORDER);
>
> Indent messed up. Feel free to exceed 80 chars if it aids readability.
Fixed!
>
> > if (result != SCAN_SUCCEED)
> > goto out_nolock;
> > }
> > @@ -1197,7 +1210,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> > pte = pte_offset_map_lock(mm, &_pmd, address, &pte_ptl);
> > if (pte) {
> > result = __collapse_huge_page_isolate(vma, address, pte, cc,
> > - &compound_pagelist);
> > + &compound_pagelist, HPAGE_PMD_ORDER);
>
> Dito.
Fixed!
>
>
> Apart from that, nothing jumped at me
>
> Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Thanks for the ack! I fixed the compile issue you noted too.
>
> --
> Cheers,
>
> David / dhildenb
>
