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

[Baolin Wang]

On 2/9/26 5:55 PM, David Hildenbrand (Arm) wrote:
> 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 ? :)

Sure, I can take a close look and address that in my follow-up patchset.