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Message-ID: <6a6f03df-8dc4-44aa-ad25-40f51ad98266@arm.com>
Date: Fri, 19 Sep 2025 09:41:35 +0530
From: Dev Jain <dev.jain@....com>
To: David Hildenbrand <david@...hat.com>, Lance Yang <lance.yang@...ux.dev>,
akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com
Cc: ziy@...dia.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com,
npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
ioworker0@...il.com, kirill@...temov.name, hughd@...gle.com,
mpenttil@...hat.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard
PTEs
On 19/09/25 12:17 am, David Hildenbrand wrote:
> On 18.09.25 07:04, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.dev>
>>
>> Guard PTE markers are installed via MADV_GUARD_INSTALL to create
>> lightweight guard regions.
>>
>> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail
>> when
>> encountering such a range.
>>
>> MADV_COLLAPSE fails deep inside the collapse logic when trying to
>> swap-in
>> the special marker in __collapse_huge_page_swapin().
>>
>> hpage_collapse_scan_pmd()
>> `- collapse_huge_page()
>> `- __collapse_huge_page_swapin() -> fails!
>>
>> khugepaged's behavior is slightly different due to its max_ptes_swap
>> limit
>> (default 64). It won't fail as deep, but it will still needlessly
>> scan up
>> to 64 swap entries before bailing out.
>>
>> IMHO, we can and should detect this much earlier.
>>
>> This patch adds a check directly inside the PTE scan loop. If a guard
>> marker is found, the scan is aborted immediately with
>> SCAN_PTE_NON_PRESENT,
>> avoiding wasted work.
>>
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>> ---
>> mm/khugepaged.c | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 9ed1af2b5c38..70ebfc7c1f3e 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct
>> mm_struct *mm,
>> result = SCAN_PTE_UFFD_WP;
>> goto out_unmap;
>> }
>> + /*
>> + * Guard PTE markers are installed by
>> + * MADV_GUARD_INSTALL. Any collapse path must
>> + * not touch them, so abort the scan immediately
>> + * if one is found.
>> + */
>> + if (is_guard_pte_marker(pteval)) {
>> + result = SCAN_PTE_NON_PRESENT;
>> + goto out_unmap;
>> + }
>
> Thinking about it, this is interesting.
>
> Essentially we track any non-swap swap entries towards
> khugepaged_max_ptes_swap, which is rather weird.
>
> I think we might also run into migration entries here and hwpoison
> entries?
>
> So what about just generalizing this:
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index af5f5c80fe4ed..28f1f4bf0e0a8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1293,7 +1293,24 @@ static int hpage_collapse_scan_pmd(struct
> mm_struct *mm,
> for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR;
> _pte++, _address += PAGE_SIZE) {
> pte_t pteval = ptep_get(_pte);
> - if (is_swap_pte(pteval)) {
> +
> + 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) {
> @@ -1313,18 +1330,7 @@ static int hpage_collapse_scan_pmd(struct
> mm_struct *mm,
> goto out_unmap;
> }
> }
> - 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;
> - }
> - }
> +
> if (pte_uffd_wp(pteval)) {
> /*
> * Don't collapse the page if any of the small
>
>
> With that, the function flow looks more similar to
> __collapse_huge_page_isolate(),
> except that we handle swap entries in there now.
This looks good!
>
>
> And with that in place, couldn't we factor out a huge chunk of both
> scanning
> functions into some helper (passing whether swap entries are allowed
> or not?).
>
> Yes, I know, refactoring khugepaged, crazy idea.
>
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