Re: [PATCH] mm/page_vma_mapped_walk: add missing pgtable entry accessors

[Alexander Gordeev]

On Mon, Apr 27, 2026 at 11:02:50AM +0200, David Hildenbrand (Arm) wrote:
> On 4/27/26 07:20, Alexander Gordeev wrote:
> > Convert pgtable direct entry dereferences to the corresponding
> > pXdp_get() accessors. Use ptep_get_lockless() variant for PTE
> > reads when no lock is taken.
> > 
> > Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
> > ---
> >  mm/page_vma_mapped.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index b38a1d00c971..a4520bb10d2a 100644
> > --- a/mm/page_vma_mapped.c
> > +++ b/mm/page_vma_mapped.c
> > @@ -41,7 +41,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw, pmd_t *pmdvalp,
> >  	if (!pvmw->pte)
> >  		return false;
> >  
> > -	ptent = ptep_get(pvmw->pte);
> > +	ptent = ptep_get_lockless(pvmw->pte);

(*)

> >  	if (pte_none(ptent)) {
> >  		return false;
> > @@ -219,17 +219,17 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> >  restart:
> >  	do {
> >  		pgd = pgd_offset(mm, pvmw->address);
> > -		if (!pgd_present(*pgd)) {
> > +		if (!pgd_present(pgdp_get(pgd))) {
> >  			step_forward(pvmw, PGDIR_SIZE);
> >  			continue;
> >  		}
> >  		p4d = p4d_offset(pgd, pvmw->address);
> > -		if (!p4d_present(*p4d)) {
> > +		if (!p4d_present(p4dp_get(p4d))) {
> >  			step_forward(pvmw, P4D_SIZE);
> >  			continue;
> >  		}
> >  		pud = pud_offset(p4d, pvmw->address);
> > -		if (!pud_present(*pud)) {
> > +		if (!pud_present(pudp_get(pud))) {
> >  			step_forward(pvmw, PUD_SIZE);
> >  			continue;
> 
> Wasn't there a problem with folded page tables, where we would no longer be able
> to optimize out the folded page table accesses?

Ok, that is a different topic. I actually tried to resolve the lockless
issue(s) in (*) while the direct dereferences just seemed to be relevant
to go along (similarly to the GUP patch).

But I would rather resend ptep_get() => ptep_get_lockless() fixes for
this patch.

Would it work?

> I thought we discussed ways to resolve that. Let me dig ...
> 
> 
> -- 
> Cheers,
> 
> David

Thanks!