Re: [PATCH v2 1/3] Revert "mm/ksm: convert break_ksm() from walk_page_range_vma() to folio_walk"

[David Hildenbrand (Red Hat)]

On 05.11.25 14:28, Pedro Demarchi Gomes wrote:
> On Mon, Nov 03, 2025 at 06:00:08PM +0100, David Hildenbrand (Red Hat) wrote:
> > On 31.10.25 18:46, Pedro Demarchi Gomes wrote:
> > > This reverts commit e317a8d8b4f600fc7ec9725e26417030ee594f52 and changes
> > > function break_ksm_pmd_entry() to use folios.
> > >
> > > This reverts break_ksm() to use walk_page_range_vma() instead of
> > > folio_walk_start().
> > > This will make it easier to later modify break_ksm() to perform a proper
> > > range walk.
> > >
> > > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > > Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@xxxxxxxxx>
> > > ---
> > >    mm/ksm.c | 63 ++++++++++++++++++++++++++++++++++++++++++--------------
> > >    1 file changed, 47 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/mm/ksm.c b/mm/ksm.c
> > > index 4f672f4f2140..922d2936e206 100644
> > > --- a/mm/ksm.c
> > > +++ b/mm/ksm.c
> > > @@ -607,6 +607,47 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
> > >    	return atomic_read(&mm->mm_users) == 0;
> > >    }
> > > +static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned long next,
> > > +			struct mm_walk *walk)
> > > +{
> > > +	struct folio *folio = NULL;
> > > +	spinlock_t *ptl;
> > > +	pte_t *pte;
> > > +	pte_t ptent;
> > > +	int ret;
> > > +
> > > +	pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> > > +	if (!pte)
> > > +		return 0;
> > > +	ptent = ptep_get(pte);
> > > +	if (pte_present(ptent)) {
> > > +		folio = vm_normal_folio(walk->vma, addr, ptent);
> > > +	} else if (!pte_none(ptent)) {
> > > +		swp_entry_t entry = pte_to_swp_entry(ptent);
> > > +
> > > +		/*
> > > +		 * As KSM pages remain KSM pages until freed, no need to wait
> > > +		 * here for migration to end.
> > > +		 */
> > > +		if (is_migration_entry(entry))
> > > +			folio = pfn_swap_entry_folio(entry);
> > > +	}
> > > +	/* return 1 if the page is an normal ksm page or KSM-placed zero page */
> > > +	ret = (folio && folio_test_ksm(folio)) || is_ksm_zero_pte(ptent);
> >
> > Staring again, we should really call is_ksm_zero_pte() only if we know the
> > folio is present.
> >
> > It's not super dangerous in the old code (because we would only look at
> > present an migration entries), but now you are making it possible to call it
> > on even more non-present ptes.
>
> IIUC vm_normal_folio will return NULL in the case of a ksm zero pte, so
> we can not do
> 	found = folio && (folio_test_ksm(folio) || is_ksm_zero_pte(pte))
> because it will always be false for a ksm zero pte.
> So we should do
> 	found = (folio && folio_test_ksm(folio)) || (pte_present(ptent)
> 		&& is_ksm_zero_pte(ptent));
> since if the pte is present and is a zero pte we can guarantee that
> the folio is present.

Yes exactly.


--
Cheers

David