Re: [PATCH v2] mm/page_vma_mapped_walk: Use ptep_get_lockless() for lockless access

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

On Fri, May 08, 2026 at 10:17:16AM +0200, David Hildenbrand (Arm) wrote:
> >> So the rule here is:
> >>
> >>   * ptep_get_lockless() could be used for locked and not locked
> >>   * ptep_get() only used when locked
> >>
> >> Right?
> > 
> > Yes, this is my assumption.
> 
> I agree, ptep_get_lockless() simply makes sense to return something sensible if
> there are concurrent modifications (which cannot happen when the PTL is held).
> 
> That's why only 32bit with 64bit PTEs and arm64 even has to special-case it.
> 
> 
> We should clarify in the patch description that in the do-while loop, we might
> or might not hold the PTL, and that calling ptep_get_lockless() with the PTL
> held is OK.
> 
> I wonder if it's more efficient and clearer, to use the correct variant, though?
> 
> 
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index a4d52fdb3056..36d97661a4e5 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -187,6 +187,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>         p4d_t *p4d;
>         pud_t *pud;
>         pmd_t pmde;
> +       pte_t pteval;
> 
>         /* The only possible pmd mapping has been handled on last iteration */
>         if (pvmw->pmd && !pvmw->pte)
> @@ -310,7 +311,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
>                                 goto restart;
>                         }
>                         pvmw->pte++;
> -               } while (pte_none(ptep_get(pvmw->pte)));
> +                       if (!pvmw->ptl)
> +                               pteval = ptep_get_lockless(pvmw->pte);
> +                       else
> +                               pteval = ptep_get(pvmw->pte);
> +               } while (pte_none(pteval));

Looks fine to me. I will try and add it to the next version.

>                 if (!pvmw->ptl) {
>                         spin_lock(ptl);
> 
> 
> -- 
> Cheers,
> 
> David

Thanks!