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

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Message-ID: <f7392f43-b8f1-4e6a-b9c8-25ad8a47f82c@lucifer.local>
Date: Tue, 14 Oct 2025 15:39:21 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, david@...hat.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,
        richard.weiyang@...il.com
Subject: Re: [PATCH mm-new v3 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries

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...

>
> > 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 ...

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'.

>
> >
> > > +				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 :)

>
> >
> > 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