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

Pedro Demarchi Gomes
2025-11-05T13:30:49Z

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@redhat.com>
> > Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
> > ---
> > 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.

Sorry if I am missing something.

> --
> Cheers
>
> David
>