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

Alexander Gordeev
2026-05-03T06:51:31Z

On Fri, May 01, 2026 at 09:01:09PM +0200, David Hildenbrand (Arm) wrote:
> On 4/29/26 15:47, Alexander Gordeev wrote:
> > 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@linux.ibm.com>
> >>> ---
> >>> 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?
>
> You mean only sending {*) ?

Only this:

diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
index a4d52fdb3056..6559e17f11c2 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;
@@ -310,7 +310,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
goto restart;
}
pvmw->pte++;
- } while (pte_none(ptep_get(pvmw->pte)));
+ } while (pte_none(ptep_get_lockless(pvmw->pte)));

if (!pvmw->ptl) {
spin_lock(ptl);

> --
> Cheers,
>
> David