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

[Lance Yang <lance.yang@linux.dev>]

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))) {
		[...]
	}
[...]}
```

> 
>>
>>>
>>>> +				result = SCAN_PTE_NON_PRESENT;
>>>> +				goto out_unmap;
>>>> +			}
>>>> +
>>>>    			++unmapped;
>>>>    			if (!cc->is_khugepaged ||
>>>>    			    unmapped <= khugepaged_max_ptes_swap) {
>>>> @@ -1290,7 +1311,7 @@ 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)) {
>>>
>>> Again you're assuming it's a swap entry but you're not asserting this is a swap
>>> entry in this branch?
>>
>> As we discussed above, the non_swap_entry() check has already kicked out
>> anything that isn't a true swap entry, right?
> 
> This is a different function?
> 
> Actually I'm mistaken here I think - you check in the code above:
> 
> 		if (is_swap_pte(pteval)) {
> 			...
> 		}
> 
> So this is fine, please ignore sorry :)

No worries at all, thanks for double-checking and clarifying!

> 
>>
>>>
>>> Also an aside - I hate, hate, hate how this uffd wp stuff has infiltrated all
>>> kinds of open-coded stuff. It's so gross (not your fault, just a general
>>> comment...)
>>
>> Haha, tell me about it. No argument from me there ;)
> 
> :)
> 
>>
>> Thanks,
>> Lance
> 
> Cheers, Lorenzo