linux-kernel - Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <d7012c9c-f4a9-4d49-a921-2cc175f3411d@redhat.com>
Date: Mon, 29 Sep 2025 12:29:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, baohua@...nel.org,
 baolin.wang@...ux.alibaba.com, dev.jain@....com, hughd@...gle.com,
 ioworker0@...il.com, kirill@...temov.name, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, mpenttil@...hat.com, npache@...hat.com,
 ryan.roberts@....com, ziy@...dia.com, richard.weiyang@...il.com,
 akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new 1/1] mm/khugepaged: abort collapse scan on non-swap
 entries

On 24.09.25 13:47, Lance Yang wrote:
> 
> 
> On 2025/9/24 18:10, David Hildenbrand wrote:
>> On 24.09.25 12:02, Lance Yang wrote:
>>> From: Lance Yang <lance.yang@...ux.dev>
>>>
>>> The existing check in hpage_collapse_scan_pmd() is specific to uffd-wp
>>> markers. Other special markers (e.g., GUARD, POISONED) would not be
>>> caught
>>> early, leading to failures deeper in the swap-in logic.
>>>
>>> hpage_collapse_scan_pmd()
>>>    `- collapse_huge_page()
>>>        `- __collapse_huge_page_swapin() -> fails!
>>>
>>> As David suggested[1], this patch skips any such non-swap entries early.
>>> If a special marker is found, the scan is aborted immediately with the
>>> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
>>> work.
>>
>> Note that I suggested to skip all non-present entries except swap
>> entries, which includes migration entries, hwpoisoned entries etc.
> 
> Oops, I completely misunderstood your suggestion :(
> 
> It should be to handle all special non-present entries (migration,
> hwpoison, markers), not just a specific type of marker ...
> 
> How about this version, which handles all non-swap entries as you
> suggested?
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 7ab2d1a42df3..27f432e7f07c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1284,7 +1284,23 @@ 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 (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) {
> @@ -1293,7 +1309,7 @@ static int hpage_collapse_scan_pmd(struct
> mm_struct *mm,
>                                    * enabled swap entries.  Please see
>                                    * comment below for pte_uffd_wp().
>                                    */
> -                               if (pte_swp_uffd_wp_any(pteval)) {
> +                               if (pte_swp_uffd_wp(pteval)) {
>                                           result = SCAN_PTE_UFFD_WP;
>                                           goto out_unmap;
>                                   }
> @@ -1304,18 +1320,6 @@ 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)) {

 From a quick glimpse, this should work. And as raised, we might be able 
to unify later the scanning with the almost-duplicated code when we do 
the second scan.

-- 
Cheers

David / dhildenb