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Message-ID: <8e7c7078-7757-44d2-b44a-aa8cf17cabd0@linux.dev>
Date: Wed, 15 Oct 2025 09:48:05 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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
On 2025/10/15 01:41, Lorenzo Stoakes wrote:
> 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?
Got it, thanks!
>
> 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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