Re: [PATCH RFC 03/14] mm: compare pfns only if the entry is present when inserting pfns/pages

[David Hildenbrand]

On 20.06.25 15:27, Oscar Salvador wrote:
> On Tue, Jun 17, 2025 at 05:43:34PM +0200, David Hildenbrand wrote:
> > Doing a pte_pfn() etc. of something that is not a present page table
> > entry is wrong. Let's check in all relevant cases where we want to
> > upgrade write permissions when inserting pfns/pages whether the entry
> > is actually present.
>
> Maybe I would add that's because the pte can have other info like
> marker, swp_entry etc.
>
> > It's not expected to have caused real harm in practice, so this is more a
> > cleanup than a fix for something that would likely trigger in some
> > weird circumstances.
> >
> > At some point, we should likely unify the two pte handling paths,
> > similar to how we did it for pmds/puds.
> >
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
> Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>
>
> Should we scream if someone passes us a non-present entry?

Probably? Good point, let me think about that.

> > ---
> >   mm/huge_memory.c | 4 ++--
> >   mm/memory.c      | 4 ++--
> >   2 files changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 8e0e3cfd9f223..e52360df87d15 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1392,7 +1392,7 @@ static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
> >   		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
> >   					  fop.pfn;
> >   
> > -		if (write) {
> > +		if (write && pmd_present(*pmd)) {
> >   			if (pmd_pfn(*pmd) != pfn) {
> >   				WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
> >   				return -EEXIST;
> > @@ -1541,7 +1541,7 @@ static void insert_pud(struct vm_area_struct *vma, unsigned long addr,
> >   		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
> >   					  fop.pfn;
> >   
> > -		if (write) {
> > +		if (write && pud_present(*pud)) {
> >   			if (WARN_ON_ONCE(pud_pfn(*pud) != pfn))
> >   				return;
> >   			entry = pud_mkyoung(*pud);
> > diff --git a/mm/memory.c b/mm/memory.c
> > index a1b5575db52ac..9a1acd057ce59 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2137,7 +2137,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
> >   	pte_t pteval = ptep_get(pte);
> >   
> >   	if (!pte_none(pteval)) {
> > -		if (!mkwrite)
> > +		if (!mkwrite || !pte_present(pteval))
> >   			return -EBUSY;
>
> Why EBUSY? because it might transitory?

I was confused myself about error handling, and why it differs for all 
cases ... adding to me todo list to investigate that (clean it up ...) :)

--
Cheers,

David / dhildenb