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Message-ID: <703b22dc-0123-42ba-9984-074b9cda468c@linux.dev>
Date: Mon, 27 Oct 2025 17:17:28 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Nico Pache <npache@...hat.com>
Cc: david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, ryan.roberts@....com,
dev.jain@....com, corbet@....net, rostedt@...dmis.org, mhiramat@...nel.org,
mathieu.desnoyers@...icios.com, akpm@...ux-foundation.org,
linux-kernel@...r.kernel.org, baohua@...nel.org, willy@...radead.org,
peterx@...hat.com, wangkefeng.wang@...wei.com, linux-mm@...ck.org,
usamaarif642@...il.com, sunnanyong@...wei.com, vishal.moola@...il.com,
thomas.hellstrom@...ux.intel.com, yang@...amperecomputing.com,
kas@...nel.org, aarcange@...hat.com, raquini@...hat.com,
anshuman.khandual@....com, catalin.marinas@....com, tiwai@...e.de,
will@...nel.org, dave.hansen@...ux.intel.com, jack@...e.cz, cl@...two.org,
jglisse@...gle.com, surenb@...gle.com, zokeefe@...gle.com,
hannes@...xchg.org, rientjes@...gle.com, mhocko@...e.com,
rdunlap@...radead.org, hughd@...gle.com, richard.weiyang@...il.com,
vbabka@...e.cz, rppt@...nel.org, jannh@...gle.com, pfalcato@...e.de,
linux-trace-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v12 mm-new 05/15] khugepaged: generalize
__collapse_huge_page_* for mTHP support
On 2025/10/23 02:37, Nico Pache wrote:
> generalize the order of the __collapse_huge_page_* functions
> to support future mTHP collapse.
>
> mTHP collapse will not honor the khugepaged_max_ptes_shared or
> khugepaged_max_ptes_swap parameters, and will fail if it encounters a
> shared or swapped entry.
Yeah, IMHO, it's the right call to avoid the complexity of potential
"collapse creep" at this stage and get the core functionality right first ;)
>
> No functional changes in this patch.
>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Acked-by: David Hildenbrand <david@...hat.com>
> Co-developed-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
Cool! LGTM.
Reviewed-by: Lance Yang <lance.yang@...ux.dev>
> mm/khugepaged.c | 78 ++++++++++++++++++++++++++++++-------------------
> 1 file changed, 48 insertions(+), 30 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 36ee659acfbb..4ccebf5dda97 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -537,25 +537,25 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> }
>
> static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> - unsigned long start_addr,
> - pte_t *pte,
> - struct collapse_control *cc,
> - struct list_head *compound_pagelist)
> + unsigned long start_addr, pte_t *pte, struct collapse_control *cc,
> + unsigned int order, struct list_head *compound_pagelist)
> {
> struct page *page = NULL;
> struct folio *folio = NULL;
> unsigned long addr = start_addr;
> pte_t *_pte;
> int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
> + const unsigned long nr_pages = 1UL << order;
> + int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
>
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> + for (_pte = pte; _pte < pte + nr_pages;
> _pte++, addr += PAGE_SIZE) {
> pte_t pteval = ptep_get(_pte);
> if (pte_none_or_zero(pteval)) {
> ++none_or_zero;
> if (!userfaultfd_armed(vma) &&
> (!cc->is_khugepaged ||
> - none_or_zero <= khugepaged_max_ptes_none)) {
> + none_or_zero <= max_ptes_none)) {
> continue;
> } else {
> result = SCAN_EXCEED_NONE_PTE;
> @@ -583,8 +583,14 @@ 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.
> + */
> + 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;
> @@ -677,18 +683,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> }
>
> 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 vm_area_struct *vma, unsigned long address,
> + spinlock_t *ptl, unsigned int order,
> + struct list_head *compound_pagelist)
> {
> - 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;
> + const unsigned long nr_pages = 1UL << order;
>
> - for (_pte = pte; _pte < pte + HPAGE_PMD_NR; _pte += nr_ptes,
> + for (_pte = pte; _pte < pte + nr_pages; _pte += nr_ptes,
> address += nr_ptes * PAGE_SIZE) {
> nr_ptes = 1;
> pteval = ptep_get(_pte);
> @@ -741,13 +747,11 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
> }
>
> 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)
> + pmd_t *pmd, pmd_t orig_pmd, struct vm_area_struct *vma,
> + unsigned int order, struct list_head *compound_pagelist)
> {
> spinlock_t *pmd_ptl;
> -
> + const unsigned long nr_pages = 1UL << order;
> /*
> * Re-establish the PMD to point to the original page table
> * entry. Restoring PMD needs to be done prior to releasing
> @@ -761,7 +765,7 @@ 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 + nr_pages, compound_pagelist);
> }
>
> /*
> @@ -781,16 +785,16 @@ 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,
> + unsigned long address, spinlock_t *ptl, unsigned int order,
> struct list_head *compound_pagelist)
> {
> unsigned int i;
> int result = SCAN_SUCCEED;
> -
> + const unsigned long nr_pages = 1UL << order;
> /*
> * Copying pages' contents is subject to memory poison at any iteration.
> */
> - for (i = 0; i < HPAGE_PMD_NR; i++) {
> + for (i = 0; i < nr_pages; i++) {
> pte_t pteval = ptep_get(pte + i);
> struct page *page = folio_page(folio, i);
> unsigned long src_addr = address + i * PAGE_SIZE;
> @@ -809,10 +813,10 @@ 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);
> + order, compound_pagelist);
> else
> __collapse_huge_page_copy_failed(pte, pmd, orig_pmd, vma,
> - compound_pagelist);
> + order, compound_pagelist);
>
> return result;
> }
> @@ -985,13 +989,12 @@ static int check_pmd_still_valid(struct mm_struct *mm,
> * Returns result: if not SCAN_SUCCEED, mmap_lock has been released.
> */
> static int __collapse_huge_page_swapin(struct mm_struct *mm,
> - struct vm_area_struct *vma,
> - unsigned long start_addr, pmd_t *pmd,
> - int referenced)
> + struct vm_area_struct *vma, unsigned long start_addr,
> + pmd_t *pmd, int referenced, unsigned int order)
> {
> int swapped_in = 0;
> vm_fault_t ret = 0;
> - unsigned long addr, end = start_addr + (HPAGE_PMD_NR * PAGE_SIZE);
> + unsigned long addr, end = start_addr + (PAGE_SIZE << order);
> int result;
> pte_t *pte = NULL;
> spinlock_t *ptl;
> @@ -1022,6 +1025,19 @@ 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);
> @@ -1142,7 +1158,7 @@ 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;
> }
> @@ -1190,6 +1206,7 @@ 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,
> + HPAGE_PMD_ORDER,
> &compound_pagelist);
> spin_unlock(pte_ptl);
> } else {
> @@ -1220,6 +1237,7 @@ 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,
> + HPAGE_PMD_ORDER,
> &compound_pagelist);
> pte_unmap(pte);
> if (unlikely(result != SCAN_SUCCEED))
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