Re: [PATCH v3 1/3] mm/mempolicy: skip non-present PMDs when queueing folios

[Gregory Price]

On Fri, Jul 10, 2026 at 03:55:21AM -0700, Usama Arif wrote:
> queue_folios_pmd() is called under pmd_trans_huge_lock(), whose
> pmd_is_huge() check returns true for any non-present, non-none PMD
> softleaf. Passing such a PMD to pmd_folio() treats the softleaf encoding
> as a hardware PFN and can return a bogus folio pointer.
> 
> Mirror queue_folios_pte_range(): handle non-present entries before
> looking up a folio. Keep migration entries counted as failures, but skip
> other non-present PMDs such as device-private entries.
> 
> Potential trigger: an HMM-based GPU driver migrates an anonymous THP
> folio to device memory via migrate_vma_pages(), leaving a device-private
> PMD. Userspace then calls mbind(), migrate_pages() or
> set_mempolicy_home_node() on that range.
> 
> Reported-by: sashiko-bot <sashiko-bot@xxxxxxxxxx>
> Link: https://sashiko.dev/#/patchset/20260703173903.3789516-1-usama.arif%40linux.dev?part=6
> Fixes: 368076f52ebe ("mm/huge_memory: add device-private THP support to PMD operations")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

Reviewed-by: Gregory Price <gourry@xxxxxxxxxx>