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Message-ID: <04734f2d-ea1f-487c-b01c-c3d6e1b4a778@lucifer.local>
Date: Tue, 14 Oct 2025 18:41:06 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
npache@...hat.com, ryan.roberts@....com, dev.jain@....com,
baohua@...nel.org, ioworker0@...il.com, richard.weiyang@...il.com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new v3 3/3] mm/khugepaged: merge PTE scanning logic
into a new helper
OK so this patch _is_ based on [0] already being applied then?
I don't see any is_swap_pte() here so presumably so right?
Can we just respin these series and put them together? Because now I've
reviewed a bunch of stuff in [0], which this series depends upon, and
you're saying we should now review on this instead of that and it's a bit
of a mess.
Once review here is dealt with can you combine [0] into this series please?
For [0] I would like you to reinstate the is_swap_pte() conditional as
(copiously :) discussed over there.
[0}: https://lore.kernel.org/all/20251008032657.72406-1-lance.yang@linux.dev/
Thanks, Lorenzo
On Wed, Oct 08, 2025 at 12:37:48PM +0800, Lance Yang wrote:
> From: Lance Yang <lance.yang@...ux.dev>
>
> As David suggested, the PTE scanning logic in hpage_collapse_scan_pmd()
> and __collapse_huge_page_isolate() was almost duplicated.
>
> This patch cleans things up by moving all the common PTE checking logic
> into a new shared helper, thp_collapse_check_pte(). While at it, we use
> vm_normal_folio() instead of vm_normal_page().
>
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Dev Jain <dev.jain@....com>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
> ---
> mm/khugepaged.c | 243 ++++++++++++++++++++++++++----------------------
> 1 file changed, 130 insertions(+), 113 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b5c0295c3414..7116caae1fa4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -61,6 +61,12 @@ enum scan_result {
> SCAN_PAGE_FILLED,
> };
>
> +enum pte_check_result {
> + PTE_CHECK_SUCCEED,
> + PTE_CHECK_CONTINUE,
> + PTE_CHECK_FAIL,
> +};
> +
> #define CREATE_TRACE_POINTS
> #include <trace/events/huge_memory.h>
>
> @@ -533,62 +539,139 @@ static void release_pte_pages(pte_t *pte, pte_t *_pte,
> }
> }
>
> +/*
> + * thp_collapse_check_pte - Check if a PTE is suitable for THP collapse
> + * @pte: The PTE to check
> + * @vma: The VMA the PTE belongs to
> + * @addr: The virtual address corresponding to this PTE
> + * @foliop: On success, used to return a pointer to the folio
> + * Must be non-NULL
> + * @none_or_zero: Counter for none/zero PTEs. Must be non-NULL
> + * @unmapped: Counter for swap PTEs. Can be NULL if not scanning swaps
> + * @shared: Counter for shared pages. Must be non-NULL
> + * @scan_result: Used to return the failure reason (SCAN_*) on a
> + * PTE_CHECK_FAIL return. Must be non-NULL
> + * @cc: Collapse control settings
> + *
> + * Returns:
> + * PTE_CHECK_SUCCEED - PTE is suitable, proceed with further checks
> + * PTE_CHECK_CONTINUE - Skip this PTE and continue scanning
> + * PTE_CHECK_FAIL - Abort collapse scan
> + */
> +static inline int thp_collapse_check_pte(pte_t pte, struct vm_area_struct *vma,
> + unsigned long addr, struct folio **foliop, int *none_or_zero,
> + int *unmapped, int *shared, int *scan_result,
> + struct collapse_control *cc)
> +{
> + struct folio *folio = NULL;
> +
> + if (pte_none(pte) || is_zero_pfn(pte_pfn(pte))) {
> + (*none_or_zero)++;
> + if (!userfaultfd_armed(vma) &&
> + (!cc->is_khugepaged ||
> + *none_or_zero <= khugepaged_max_ptes_none)) {
> + return PTE_CHECK_CONTINUE;
> + } else {
> + *scan_result = SCAN_EXCEED_NONE_PTE;
> + count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> + return PTE_CHECK_FAIL;
> + }
> + } else if (!pte_present(pte)) {
> + if (!unmapped) {
> + *scan_result = SCAN_PTE_NON_PRESENT;
> + return PTE_CHECK_FAIL;
> + }
> +
> + if (non_swap_entry(pte_to_swp_entry(pte))) {
> + *scan_result = SCAN_PTE_NON_PRESENT;
> + return PTE_CHECK_FAIL;
> + }
> +
> + (*unmapped)++;
> + if (!cc->is_khugepaged ||
> + *unmapped <= khugepaged_max_ptes_swap) {
> + /*
> + * Always be strict with uffd-wp enabled swap
> + * entries. Please see comment below for
> + * pte_uffd_wp().
> + */
> + if (pte_swp_uffd_wp(pte)) {
> + *scan_result = SCAN_PTE_UFFD_WP;
> + return PTE_CHECK_FAIL;
> + }
> + return PTE_CHECK_CONTINUE;
> + } else {
> + *scan_result = SCAN_EXCEED_SWAP_PTE;
> + count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> + return PTE_CHECK_FAIL;
> + }
> + } else if (pte_uffd_wp(pte)) {
> + /*
> + * Don't collapse the page if any of the small PTEs are
> + * armed with uffd write protection. Here we can also mark
> + * the new huge pmd as write protected if any of the small
> + * ones is marked but that could bring unknown userfault
> + * messages that falls outside of the registered range.
> + * So, just be simple.
> + */
> + *scan_result = SCAN_PTE_UFFD_WP;
> + return PTE_CHECK_FAIL;
> + }
> +
> + folio = vm_normal_folio(vma, addr, pte);
> + if (unlikely(!folio) || unlikely(folio_is_zone_device(folio))) {
> + *scan_result = SCAN_PAGE_NULL;
> + return PTE_CHECK_FAIL;
> + }
> +
> + if (!folio_test_anon(folio)) {
> + VM_WARN_ON_FOLIO(true, folio);
> + *scan_result = SCAN_PAGE_ANON;
> + return PTE_CHECK_FAIL;
> + }
> +
> + /*
> + * We treat a single page as shared if any part of the THP
> + * is shared.
> + */
> + if (folio_maybe_mapped_shared(folio)) {
> + (*shared)++;
> + if (cc->is_khugepaged && *shared > khugepaged_max_ptes_shared) {
> + *scan_result = SCAN_EXCEED_SHARED_PTE;
> + count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> + return PTE_CHECK_FAIL;
> + }
> + }
> +
> + *foliop = folio;
> +
> + return PTE_CHECK_SUCCEED;
> +}
> +
> 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)
> {
> - 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;
> + int pte_check_res;
>
> for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
> _pte++, addr += PAGE_SIZE) {
> pte_t pteval = ptep_get(_pte);
> - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> - ++none_or_zero;
> - if (!userfaultfd_armed(vma) &&
> - (!cc->is_khugepaged ||
> - none_or_zero <= khugepaged_max_ptes_none)) {
> - continue;
> - } else {
> - result = SCAN_EXCEED_NONE_PTE;
> - count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> - goto out;
> - }
> - } else if (!pte_present(pteval)) {
> - result = SCAN_PTE_NON_PRESENT;
> - goto out;
> - } else if (pte_uffd_wp(pteval)) {
> - result = SCAN_PTE_UFFD_WP;
> - goto out;
> - }
> - page = vm_normal_page(vma, addr, pteval);
> - if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
> - result = SCAN_PAGE_NULL;
> - goto out;
> - }
>
> - folio = page_folio(page);
> - if (!folio_test_anon(folio)) {
> - VM_WARN_ON_FOLIO(true, folio);
> - result = SCAN_PAGE_ANON;
> - goto out;
> - }
> + pte_check_res = thp_collapse_check_pte(pteval, vma, addr,
> + &folio, &none_or_zero, NULL, &shared,
> + &result, cc);
>
> - /* See hpage_collapse_scan_pmd(). */
> - if (folio_maybe_mapped_shared(folio)) {
> - ++shared;
> - if (cc->is_khugepaged &&
> - shared > khugepaged_max_ptes_shared) {
> - result = SCAN_EXCEED_SHARED_PTE;
> - count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> - goto out;
> - }
> - }
> + if (pte_check_res == PTE_CHECK_CONTINUE)
> + continue;
> + else if (pte_check_res == PTE_CHECK_FAIL)
> + goto out;
>
> if (folio_test_large(folio)) {
> struct folio *f;
> @@ -1264,11 +1347,11 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> pte_t *pte, *_pte;
> int result = SCAN_FAIL, referenced = 0;
> int none_or_zero = 0, shared = 0;
> - struct page *page = NULL;
> struct folio *folio = NULL;
> unsigned long addr;
> spinlock_t *ptl;
> int node = NUMA_NO_NODE, unmapped = 0;
> + int pte_check_res;
>
> VM_BUG_ON(start_addr & ~HPAGE_PMD_MASK);
>
> @@ -1287,81 +1370,15 @@ static int hpage_collapse_scan_pmd(struct mm_struct *mm,
> for (addr = start_addr, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> _pte++, addr += PAGE_SIZE) {
> pte_t pteval = ptep_get(_pte);
> - if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> - ++none_or_zero;
> - if (!userfaultfd_armed(vma) &&
> - (!cc->is_khugepaged ||
> - none_or_zero <= khugepaged_max_ptes_none)) {
> - continue;
> - } else {
> - result = SCAN_EXCEED_NONE_PTE;
> - count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> - goto out_unmap;
> - }
> - } else if (!pte_present(pteval)) {
> - if (non_swap_entry(pte_to_swp_entry(pteval))) {
> - result = SCAN_PTE_NON_PRESENT;
> - goto out_unmap;
> - }
>
> - ++unmapped;
> - if (!cc->is_khugepaged ||
> - unmapped <= khugepaged_max_ptes_swap) {
> - /*
> - * Always be strict with uffd-wp
> - * enabled swap entries. Please see
> - * comment below for pte_uffd_wp().
> - */
> - if (pte_swp_uffd_wp(pteval)) {
> - result = SCAN_PTE_UFFD_WP;
> - goto out_unmap;
> - }
> - continue;
> - } else {
> - result = SCAN_EXCEED_SWAP_PTE;
> - count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> - goto out_unmap;
> - }
> - } else if (pte_uffd_wp(pteval)) {
> - /*
> - * Don't collapse the page if any of the small
> - * PTEs are armed with uffd write protection.
> - * Here we can also mark the new huge pmd as
> - * write protected if any of the small ones is
> - * marked but that could bring unknown
> - * userfault messages that falls outside of
> - * the registered range. So, just be simple.
> - */
> - result = SCAN_PTE_UFFD_WP;
> - goto out_unmap;
> - }
> + pte_check_res = thp_collapse_check_pte(pteval, vma, addr,
> + &folio, &none_or_zero, &unmapped,
> + &shared, &result, cc);
>
> - page = vm_normal_page(vma, addr, pteval);
> - if (unlikely(!page) || unlikely(is_zone_device_page(page))) {
> - result = SCAN_PAGE_NULL;
> - goto out_unmap;
> - }
> - folio = page_folio(page);
> -
> - if (!folio_test_anon(folio)) {
> - VM_WARN_ON_FOLIO(true, folio);
> - result = SCAN_PAGE_ANON;
> + if (pte_check_res == PTE_CHECK_CONTINUE)
> + continue;
> + else if (pte_check_res == PTE_CHECK_FAIL)
> goto out_unmap;
> - }
> -
> - /*
> - * We treat a single page as shared if any part of the THP
> - * is shared.
> - */
> - if (folio_maybe_mapped_shared(folio)) {
> - ++shared;
> - if (cc->is_khugepaged &&
> - shared > khugepaged_max_ptes_shared) {
> - result = SCAN_EXCEED_SHARED_PTE;
> - count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> - goto out_unmap;
> - }
> - }
>
> /*
> * Record which node the original page is from and save this
> --
> 2.49.0
>
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