linux-kernel - Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <c44e198d-7d46-491e-adc1-86cc306c27db@redhat.com>
Date: Tue, 14 Oct 2025 17:12:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <lance.yang@...ux.dev>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: akpm@...ux-foundation.org, 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
Subject: Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries

On 14.10.25 17:01, Lance Yang wrote:
> 
> 
> On 2025/10/14 22:39, Lorenzo Stoakes wrote:
>> On Tue, Oct 14, 2025 at 10:26:20PM +0800, Lance Yang wrote:
>>>
>>>
>>> On 2025/10/14 19:08, Lorenzo Stoakes wrote:
>>>> On Wed, Oct 08, 2025 at 11:26:57AM +0800, Lance Yang wrote:
>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>>> index abe54f0043c7..bec3e268dc76 100644
>>>>> --- a/mm/khugepaged.c
>>>>> +++ b/mm/khugepaged.c
>>>>> @@ -1020,6 +1020,11 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
>>>>>     		if (!is_swap_pte(vmf.orig_pte))
>>>>>     			continue;
>>>>>
>>>>> +		if (non_swap_entry(pte_to_swp_entry(vmf.orig_pte))) {
>>>>> +			result = SCAN_PTE_NON_PRESENT;
>>>>> +			goto out;
>>>>> +		}
>>>>
>>>> OK seems in line with what we were discussing before...
>>>
>>> Yep. That's the idea :)
>>>
>>>>
>>>>> +
>>>>>     		vmf.pte = pte;
>>>>>     		vmf.ptl = ptl;
>>>>>     		ret = do_swap_page(&vmf);
>>>>> @@ -1281,7 +1286,23 @@ 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))) {
>>>>
>>>
>>> Thanks for pointing that out!
>>
>> You've deleted what I've said here and also not indicated whether you'll do what
>> I asked :)
>>
>> Please be clearer...
> 
> Oh, I didn't delete your comment at all ... It's just below ...
> 
>>
>>>>>> Hm but can't this be pte_protnone() at this stage (or something
> else)? And then <-- Here!
>>>
>>> Yeah. The funny thing is, a protnone pte cannot actually get here, IIUC.
>>>
>>> ```
>>> static inline int pte_protnone(pte_t pte)
>>> {
>>> 	return (pte_flags(pte) & (_PAGE_PROTNONE | _PAGE_PRESENT))
>>> 		== _PAGE_PROTNONE;
>>> }
>>>
>>> static inline int pte_present(pte_t a)
>>> {
>>> 	return pte_flags(a) & (_PAGE_PRESENT | _PAGE_PROTNONE);
>>> }
>>> ```
>>>
>>> On x86, pte_present() returns true for a protnone pte. And I'd assume
>>> other archs behave similarly ...
>>
>> This was one example, we may make changes in the future that result in entries
>> that are non-present but also non-swap.
>>
>> I don't see the point in eliminating this check based on an implicit, open-coded
>> assumption that this can never be the case, this is just asking for trouble.
>>
>>>
>>>> we're just assuming pte_to_swp_entry() is operating on a swap entry when it in
>>>> fact might not be?
>>>>
>>>> Couldn't we end up with false positives here?
>>>
>>> Emm, I think we're good here and the code is doing the right thing.
>>
>> I mean sorry but just - NO - to doing swap operations based on open-coded checks
>> that you implicitly feel must imply a swap entry.
>>
>> This makes the code a lot more confusing, it opens us up to accidentally
>> breaking things in future and has little to no benefit, I don't see why we're
>> doing it.
>>
>> I don't think every little 'aha X must imply Y so just eliminate Z' idea need be
>> implemented, this feels like a sort of 'mathematical reduction of code ignoring
>> all other factors'.
> 
> Understood. Changing !pte_present() to is_swap_pte() will resolve all your
> concerns, right?
> 
> ```
> if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
> [...]
> } else if (is_swap_pte(pteval)) { <-- Here
> 	if (non_swap_entry(pte_to_swp_entry(pteval))) {
> 		[...]
> 	}
> [...]}

Can we please take a step back and make sure we are not starting to do 
stuff differently than elswehere in the kernel, please?

-- 
Cheers

David / dhildenb