Re: [PATCH 1/4] mm/damon: defensively skip zone device folios in damon_get_folio()

[Gregory Price <gourry@gourry.net>]

On Tue, Jul 28, 2026 at 05:50:36PM -0700, SJ Park wrote:
> Hi Gregory,
> 
> 
> On Tue, 28 Jul 2026 15:47:11 -0400 Gregory Price <gourry@gourry.net> wrote:
> 
> > All DAMON physical- and virtual-address operations obtain their folios
> > through damon_get_folio(). That helper already excludes ZONE_DEVICE
> > memory implicitly via pfn_to_online_page() and folio_test_lru(), but
> > this is inconsistent with other callers in mm/ which test explicitly.
> > 
> > Add an explicit folio_is_zone_device() rejection in damon_get_folio()
> > so the guarantee lives in one place and covers every caller uniformly,
> > consistent with other mm walkers that reject zone device folios.
> 
> Thank you for checking this and sharing this nice patch!
> 
> However, I'm not very sure adding a call that not really required for only
> making it looks consistent with others is the right choice.  I'm especially
> concerned if this will confuse future readers including myself.
> 
> If this is a hardening purpose, we have CONFIG_DAMON_DEBUG_SANITY for the
> purpose.  What about using it with a good comment explaining why we doing that?
> 

It's actually two fold:

  1) make zone_device more consistently handled across mm/
  2) make these checks possible to abstract out later

In the private node work - every location that we'd need to filter
folios based on private node membership is a zone_device check.

Where zone device checks are missing (like here) there's some other
implicit reason (specific to zone device) that allows it to be avoided.

I pointed out in the sashiko feedback that DAMON depends on a check
for folio->pgmap in the hotplug code.  This is all quite fragile, and
we can see that at least one patch in this series fixes an actual bug.

It seemed warranted to go audit every service and add the zone_device
checks to make it clear there's a zone device interaction - even if
presently unreachable - should some silent, implicit filter change.

The goal is to come back around and replace these with something like:

    bool folio_allows_mm_op(struct folio *folio,
                            enum node_states feature)
    {
        if (folio_is_zone_device(folio) ||
            !node_state(folio_nid(folio), feature));
    }

    - if (folio_is_zone_device(folio))
    + if (folio_allows_mm_op(folio, N_MEMORY_DAMON))

If we ever get to folios having memdesc flags beyond page flags,
this also gives us a single mm-wide location for these kinds of
checks in the future.

~Gregory