Re: [PATCH v5 4/5] arm64: mm: implement the architecture-specific clear_flush_young_ptes()

[David Hildenbrand (Arm)]

On 2/9/26 10:36, Baolin Wang wrote:
> On 2/9/26 5:09 PM, David Hildenbrand (Arm) wrote:
> > On 1/29/26 02:42, Baolin Wang wrote:
> > > Indeed. I previously discussed with Ryan whether using pte_cont() was 
> > > enough, and we believed that invalid PTEs wouldn’t have the PTE_CONT 
> > > bit set. But we clearly missed the device-folio cases. Thanks for 
> > > reporting.
> > >
> > > Andrew, could you please squash the following fix into this patch? If 
> > > you prefer a new version, please let me know. Thanks.
> >
> > Isn't the real problem that we should never ever ever ever, try 
> > clearing the young bit on a non-present pte?
> >
> > See damon_ptep_mkold() how that is handled with the flushing/notify.
> >
> > There needs to be a pte_present() check in the caller.
>
> The handling of ZONE_DEVICE memory in check_pte() makes me me doubt my 
> earlier understanding. And I think you are right.
>
>      } else if (pte_present(ptent)) {
>          pfn = pte_pfn(ptent);
>      } else {
>          const softleaf_t entry = softleaf_from_pte(ptent);
>
>          /* Handle un-addressable ZONE_DEVICE memory */
>          if (!softleaf_is_device_private(entry) &&
>              !softleaf_is_device_exclusive(entry))
>              return false;
>
>          pfn = softleaf_to_pfn(entry);
>      }
>
>
> > BUT
> >
> > I recall that folio_referenced() should never apply to ZONE_DEVICE 
> > folios. folio_referenced() is only called from memory reclaim code, 
> > and ZONE_DEVICE pages never get reclaimed through vmscan.c
>
> Thanks for clarifying. So I can drop the pte valid check.

We should probably add a safety check in folio_referenced(), warning
if we would ever get a ZONE_DEVICE folio passed.

Can someone look into that ? :)

--
Cheers,

David