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 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!

> Hm but can't this be pte_protnone() at this stage (or something else)? And then

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 ...

> 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.

> 
>> +				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?

> 
> 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