Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on non-swap entries
From: David Hildenbrand
Date: Tue Oct 14 2025 - 11:12:25 EST
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 ...
else)? And then <-- Here!
Hm but can't this be pte_protnone() at this stage (or something
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