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

[Alexander Gordeev <agordeev@linux.ibm.com>]

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