Re: [PATCH v4 05/12] khugepaged: generalize __collapse_huge_page_* for mTHP support

Baolin Wang
2025-04-23T08:25:13Z

On 2025/4/23 16:00, Nico Pache wrote:
> On Wed, Apr 23, 2025 at 1:30 AM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2025/4/17 08:02, 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.
>>>
>>> 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 | 46 ++++++++++++++++++++++++++++------------------
>>> 1 file changed, 28 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>> index 883e9a46359f..5e9272ab82da 100644
>>> --- a/mm/khugepaged.c
>>> +++ b/mm/khugepaged.c
>>> @@ -565,15 +565,17 @@ 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)
>>> {
>>> 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);
>>>
>>> - 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) &&
>>> @@ -581,7 +583,7 @@ 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;
>>> @@ -609,8 +611,8 @@ 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)) {
>>> result = SCAN_EXCEED_SHARED_PTE;
>>> count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
>>> goto out;
>>> @@ -711,13 +713,14 @@ 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))) {
>>> @@ -764,7 +767,8 @@ 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;
>>>
>>> @@ -781,7 +785,7 @@ 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);
>>> }
>>>
>>> /*
>>> @@ -802,7 +806,7 @@ 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;
>>> @@ -810,7 +814,7 @@ 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;
>>> @@ -829,10 +833,10 @@ 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;
>>> }
>>> @@ -1000,11 +1004,11 @@ 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;
>>> @@ -1035,6 +1039,12 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
>>> if (!is_swap_pte(vmf.orig_pte))
>>> continue;
>>>
>>> + /* Dont swapin for mTHP collapse */
>>> + if (order != HPAGE_PMD_ORDER) {
>>> + result = SCAN_EXCEED_SWAP_PTE;
>>> + goto out;
>>> + }
>>
>> IMO, this check should move into hpage_collapse_scan_pmd(), that means
>> if we scan the swap ptes for mTHP collapse, then we can return
>> 'SCAN_EXCEED_SWAP_PTE' to abort the collapse earlier.
> I dont think this is correct. We currently abort if the global
> max_swap_ptes or max_shared_ptes is exceeded during the PMD scan.
> However if those pass (and we dont collapse at the PMD level), we will
> continue to mTHP collapses. Then during the isolate function we check
> for shared ptes in this specific mTHP range and abort if there's a
> shared ptes. For swap we only know that some pages in the PMD are
> unmapped, but we arent sure which, so we have to try and fault the
> PTEs, and if it's a swap pte, and we are on mTHP collapse, we abort
> the collapse attempt. So having swap/shared PTEs in the PMD scan, does
> not indicate that ALL mTHP collapses will fail, but some will.

Yes, you are right! I misread the code (I thought the changes were in
hpage_collapse_scan_pmd()). Sorry for the noise. Feel free to add:

Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>