linux-kernel - Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <31c3f774-edb7-420a-a6a8-3e21f2abd776@linux.dev>
Date: Sun, 5 Oct 2025 10:12:34 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Wei Yang <richard.weiyang@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, 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
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries



On 2025/10/5 09:05, Wei Yang wrote:
> On Wed, Oct 01, 2025 at 06:05:57PM +0800, Lance Yang wrote:
>>
>>
>> On 2025/10/1 16:54, Wei Yang wrote:
>>> On Wed, Oct 01, 2025 at 11:22:51AM +0800, Lance Yang wrote:
>>>> From: Lance Yang <lance.yang@...ux.dev>
>>>>
>>>> Currently, special non-swap entries (like migration, hwpoison, or PTE
>>>> markers) are not caught early in hpage_collapse_scan_pmd(), leading to
>>>> failures deep 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 any one is found, the scan is aborted immediately with the
>>>> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
>>>> work.
>>>>
>>>> [1] https://lore.kernel.org/linux-mm/7840f68e-7580-42cb-a7c8-1ba64fd6df69@redhat.com
>>>> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680-dcd55219c8bd@lucifer.local
>>>>
>>>> Suggested-by: David Hildenbrand <david@...hat.com>
>>>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>>>> ---
>>>> v1 -> v2:
>>>> - Skip all non-present entries except swap entries (per David) thanks!
>>>> - https://lore.kernel.org/linux-mm/20250924100207.28332-1-lance.yang@linux.dev/
>>>>
>>>> mm/khugepaged.c | 32 ++++++++++++++++++--------------
>>>> 1 file changed, 18 insertions(+), 14 deletions(-)
>>>>
>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>> index 7ab2d1a42df3..d0957648db19 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)) {
>>>
>>> It looks is_swap_pte() is mis-leading?
>>
>> Hmm.. not to me, IMO. is_swap_pte() just means:
>>
>> !pte_none(pte) && !pte_present(pte)
>>
> 
> Maybe it has some reason.
> 
> I took another look into __collapse_huge_page_swapin(), which just check
> is_swap_pte() before do_swap_page().
> 
> We have filtered non-swap entries in hpage_collapse_scan_pmd(), but we drop
> mmap lock before isolation. This looks we may have a chance to get non-swap
> entry.

Thanks for pointing that out!

Yep, there is a theoretical window between dropping the mmap lock
after the initial scan and re-acquiring it for isolation.

> 
> Do you think it is reasonable to add a non_swap_entry() check before
> do_swap_page()?

However, that seems unlikely in practice. IMHO, the early check in
hpage_collapse_scan_pmd() is sufficient for now, so I'd prefer to
keep it as-is :)