Re: [PATCH v10 05/13] khugepaged: generalize __collapse_huge_page_* for mTHP support

Lorenzo Stoakes
2025-08-20T14:23:17Z

On Tue, Aug 19, 2025 at 07:41:57AM -0600, 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@linux.alibaba.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Co-developed-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Dev Jain <dev.jain@arm.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> mm/khugepaged.c | 62 ++++++++++++++++++++++++++++++++++---------------
> 1 file changed, 43 insertions(+), 19 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 77e0d8ee59a0..074101d03c9d 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -551,15 +551,17 @@ 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,
> + unsigned int order)

I think it's better if we keep the output var as the last in the order. It's a
bit weird to have the order specified here.

> {
> 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_max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);

This is a weird formulation, I guess we have to go with it to keep things
consistent-ish, but it's like we have a value for this that is reliant on the
order always being PMD and then sort of awkwardly adjusting for MTHP.

I guess we're stuck with it though since we have:

/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none

I guess a more sane version of this would be a ratio or something...

Anyway probably out of scope here.

>
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> + for (_pte = pte; _pte < pte + (1 << order);

Hmm is this correct? I think shifting an int is probably a bad idea even if we
can get away with it for even PUD order atm (though... 64KB ARM hm), wouldn't
_BITUL(order) be better?

Might also be worth putting into a separate local var.

> _pte++, address += PAGE_SIZE) {
> pte_t pteval = ptep_get(_pte);
> if (pte_none(pteval) || (pte_present(pteval) &&
> @@ -567,7 +569,7 @@ 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_max_ptes_none)) {
> continue;
> } else {
> result = SCAN_EXCEED_NONE_PTE;
> @@ -595,8 +597,14 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> /* See collapse_scan_pmd(). */
> if (folio_maybe_mapped_shared(folio)) {
> ++shared;
> - if (cc->is_khugepaged &&
> - shared > khugepaged_max_ptes_shared) {
> + /*
> + * TODO: Support shared pages without leading to further
> + * mTHP collapses. Currently bringing in new pages via
> + * shared may cause a future higher order collapse on a
> + * rescan of the same range.
> + */

Can we document this if not already in a subsequent commit? :) Thanks

> + if (order != HPAGE_PMD_ORDER || (cc->is_khugepaged &&
> + shared > khugepaged_max_ptes_shared)) {
> result = SCAN_EXCEED_SHARED_PTE;
> count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> goto out;
> @@ -697,15 +705,16 @@ 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,
> + unsigned int order)
> {
> - unsigned long end = address + HPAGE_PMD_SIZE;
> + unsigned long end = address + (PAGE_SIZE << order);
> struct folio *src, *tmp;
> pte_t pteval;
> pte_t *_pte;
> unsigned int nr_ptes;
>
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> + for (_pte = pte; _pte < pte + (1 << order); _pte += nr_ptes,

Same comment as above re: 1 << order.

> address += nr_ptes * PAGE_SIZE) {
> nr_ptes = 1;
> pteval = ptep_get(_pte);
> @@ -761,7 +770,8 @@ 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,
> + unsigned int order)

Same comment as above re: parameter ordering.

> {
> spinlock_t *pmd_ptl;
>
> @@ -778,7 +788,7 @@ 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);
> }
>
> /*
> @@ -799,7 +809,7 @@ 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, unsigned int order)

Same comment as before re: parameter ordering

> {
> unsigned int i;
> int result = SCAN_SUCCEED;
> @@ -807,7 +817,7 @@ 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++) {

Same comment as before about 1 << order

> pte_t pteval = ptep_get(pte + i);
> struct page *page = folio_page(folio, i);
> unsigned long src_addr = address + i * PAGE_SIZE;
> @@ -826,10 +836,10 @@ 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;
> }
> @@ -1005,11 +1015,11 @@ 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, unsigned int 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;
> @@ -1040,6 +1050,19 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> if (!is_swap_pte(vmf.orig_pte))
> continue;
>
> + /*
> + * TODO: Support swapin without leading to further mTHP
> + * collapses. Currently bringing in new pages via swapin may
> + * cause a future higher order collapse on a rescan of the same
> + * range.
> + */
> + if (order != HPAGE_PMD_ORDER) {
> + 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);
> @@ -1160,7 +1183,7 @@ 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);
> if (result != SCAN_SUCCEED)
> goto out_nolock;
> }
> @@ -1208,7 +1231,8 @@ 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);
> spin_unlock(pte_ptl);
> } else {
> result = SCAN_PMD_NULL;
> @@ -1238,7 +1262,7 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>
> result = __collapse_huge_page_copy(pte, folio, pmd, _pmd,
> vma, address, pte_ptl,
> - &compound_pagelist);
> + &compound_pagelist, HPAGE_PMD_ORDER);
> pte_unmap(pte);
> if (unlikely(result != SCAN_SUCCEED))
> goto out_up_write;
> --
> 2.50.1
>