Re: [PATCH v3 03/16] mm: avoid unnecessary uses of is_swap_pte()

[Lorenzo Stoakes]

On Tue, Nov 11, 2025 at 09:58:36PM -0500, Zi Yan wrote:
> On 10 Nov 2025, at 17:21, Lorenzo Stoakes wrote:
>
> > There's an established convention in the kernel that we treat PTEs as
> > containing swap entries (and the unfortunately named non-swap swap entries)
> > should they be neither empty (i.e. pte_none() evaluating true) nor present
> > (i.e. pte_present() evaluating true).
> >
> > However, there is some inconsistency in how this is applied, as we also
> > have the is_swap_pte() helper which explicitly performs this check:
> >
> > 	/* check whether a pte points to a swap entry */
> > 	static inline int is_swap_pte(pte_t pte)
> > 	{
> > 		return !pte_none(pte) && !pte_present(pte);
> > 	}
> >
> > As this represents a predicate, and it's logical to assume that in order to
> > establish that a PTE entry can correctly be manipulated as a swap/non-swap
> > entry, this predicate seems as if it must first be checked.
> >
> > But we instead, we far more often utilise the established convention of
> > checking pte_none() / pte_present() before operating on entries as if they
> > were swap/non-swap.
> >
> > This patch works towards correcting this inconsistency by removing all uses
> > of is_swap_pte() where we are already in a position where we perform
> > pte_none()/pte_present() checks anyway or otherwise it is clearly logical
> > to do so.
> >
> > We also take advantage of the fact that pte_swp_uffd_wp() is only set on
> > swap entries.
> >
> > Additionally, update comments referencing to is_swap_pte() and
> > non_swap_entry().
> >
> > No functional change intended.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
> >  fs/proc/task_mmu.c            | 49 ++++++++++++++++++++++++-----------
> >  include/linux/userfaultfd_k.h |  3 +--
> >  mm/hugetlb.c                  |  6 ++---
> >  mm/internal.h                 |  6 ++---
> >  mm/khugepaged.c               | 29 +++++++++++----------
> >  mm/migrate.c                  |  2 +-
> >  mm/mprotect.c                 | 43 ++++++++++++++----------------
> >  mm/mremap.c                   |  7 +++--
> >  mm/page_table_check.c         | 13 ++++++----
> >  mm/page_vma_mapped.c          | 31 +++++++++++-----------
> >  10 files changed, 104 insertions(+), 85 deletions(-)
> >
>
> <snip>
>
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index be20468fb5a9..a4e23818f37f 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -16,6 +16,7 @@ static inline bool not_found(struct page_vma_mapped_walk *pvmw)
> >  static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> >  		    spinlock_t **ptlp)
> >  {
> > +	bool is_migration;
> >  	pte_t ptent;
> >
> >  	if (pvmw->flags & PVMW_SYNC) {
> > @@ -26,6 +27,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> >  		return !!pvmw->pte;
> >  	}
> >
> > +	is_migration = pvmw->flags & PVMW_MIGRATION;
> >  again:
> >  	/*
> >  	 * It is important to return the ptl corresponding to pte,
> > @@ -41,11 +43,14 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> >
> >  	ptent = ptep_get(pvmw->pte);
> >
> > -	if (pvmw->flags & PVMW_MIGRATION) {
> > -		if (!is_swap_pte(ptent))
>
> Here, is_migration = true and either pte_none() or pte_present()
> would return false, and ...
>
> > +	if (pte_none(ptent)) {
> > +		return false;
> > +	} else if (pte_present(ptent)) {
> > +		if (is_migration)
> >  			return false;
> > -	} else if (is_swap_pte(ptent)) {
> > +	} else if (!is_migration) {
> >  		swp_entry_t entry;
> > +
> >  		/*
> >  		 * Handle un-addressable ZONE_DEVICE memory.
> >  		 *
> > @@ -66,8 +71,6 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> >  		if (!is_device_private_entry(entry) &&
> >  		    !is_device_exclusive_entry(entry))
> >  			return false;
> > -	} else if (!pte_present(ptent)) {
> > -		return false;
>
> ... is_migration = false and !pte_present() is actually pte_none(),
> because of the is_swap_pte() above the added !is_migration check.
> So pte_none() should return false regardless of is_migration.

I guess you were working this through :) well I decided to also just to
double-check I got it right, maybe useful for you also :P -

Previously:

	if (is_migration) {
		if (!is_swap_pte(ptent))
			return false;
	} else if (is_swap_pte(ptent)) {
		... ZONE_DEVICE blah ...
	} else if (!pte_present(ptent)) {
		return false;
	}

But is_swap_pte() is the same as !pte_none() && !pte_present(), so
!is_swap_pte() is pte_none() || pte_present() by De Morgan's law:

	if (is_migration) {
		if (pte_none(ptent) || pte_present(ptent))
			return false;
	} else if (!pte_none(ptent) && !pte_present(ptent)) {
		... ZONE_DEVICE blah ...
	} else if (!pte_present(ptent)) {
		return false;
	}

In the last branch, we know (again by De Morgan's law) that either
pte_none(ptent) or pte_present(ptent).. But we explicitly check for
!pte_present(ptent) so this becomes:

	if (is_migration) {
		if (pte_none(ptent) || pte_present(ptent))
			return false;
	} else if (!pte_none(ptent) && !pte_present(ptent)) {
		... ZONE_DEVICE blah ...
	} else if (pte_none(ptent)) {
		return false;
	}

So we can generalise - regardless of is_migration, pte_none() returns false:

	if (pte_none(ptent)) {
		return false;
	} else if (is_migration) {
		if (pte_none(ptent) || pte_present(ptent))
			return false;
	} else if (!pte_none(ptent) && !pte_present(ptent)) {
		... ZONE_DEVICE blah ...
	}

Since we already check for pte_none() ahead of time, we can simplify again:

	if (pte_none(ptent)) {
		return false;
	} else if (is_migration) {
		if (pte_present(ptent))
			return false;
	} else if (!pte_present(ptent)) {
		... ZONE_DEVICE blah ...
	}

We can then put the pte_present() check in the outer branch:

	if (pte_none(ptent)) {
		return false;
	} else if (pte_present(ptent)) {
		if (is_migration)
			return false;
	} else if (!is_migration) {
		... ZONE_DEVICE blah ...
	}

Because previously an is_migration && !pte_present() case would result in no
action here.

Which is the code in this patch :)

>
> This is a nice cleanup. Thanks.
>
> >  	}
> >  	spin_lock(*ptlp);
> >  	if (unlikely(!pmd_same(*pmdvalp, pmdp_get_lockless(pvmw->pmd)))) {
> > @@ -113,21 +116,17 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw, unsigned long pte_nr)
> >  			return false;
> >
> >  		pfn = softleaf_to_pfn(entry);
> > -	} else if (is_swap_pte(ptent)) {
> > -		swp_entry_t entry;
> > +	} else if (pte_present(ptent)) {
> > +		pfn = pte_pfn(ptent);
> > +	} else {
> > +		const softleaf_t entry = softleaf_from_pte(ptent);
> >
> >  		/* Handle un-addressable ZONE_DEVICE memory */
> > -		entry = pte_to_swp_entry(ptent);
> > -		if (!is_device_private_entry(entry) &&
> > -		    !is_device_exclusive_entry(entry))
> > -			return false;
> > -
> > -		pfn = swp_offset_pfn(entry);
> > -	} else {
> > -		if (!pte_present(ptent))
>
> This !pte_present() is pte_none(). It seems that there should be

Well this should be fine though as:

		const softleaf_t entry = softleaf_from_pte(ptent);

		/* Handle un-addressable ZONE_DEVICE memory */
		if (!softleaf_is_device_private(entry) &&
		    !softleaf_is_device_exclusive(entry))
			return false;

Still correctly handles none - as softleaf_from_pte() in case of pte_none() will
be a none softleaf entry which will fail both of these tests.

So excluding pte_none() as an explicit test here was part of the rework - we no
longer have to do that.

>
> } else if (pte_none(ptent)) {
> 	return false;
> }
>
> before the above "} else {".
>
> > +		if (!softleaf_is_device_private(entry) &&
> > +		    !softleaf_is_device_exclusive(entry))
> >  			return false;
> >
> > -		pfn = pte_pfn(ptent);
> > +		pfn = softleaf_to_pfn(entry);
> >  	}
> >
> >  	if ((pfn + pte_nr - 1) < pvmw->pfn)
> > --
> > 2.51.0
>
> Otherwise, LGTM. With the above issue addressed, feel free to
> add Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Thanks!

>
> --
> Best Regards,
> Yan, Zi

Cheers, Lorenzo