mm/mempolicy: skip non-present PMDs when queueing folios

Overview

In the Linux kernel, the following vulnerability has been resolved:

mm/mempolicy: skip non-present PMDs when queueing folios

Patch series "mm: handle device-private PMDs in walk callbacks", v3.

Since commit 368076f52ebe ("mm/huge_memory: add device-private THP support to PMD operations") a PMD may hold a device-private swap entry whenever an HMM-based GPU driver migrates an anonymous THP folio to device memory via migrate_vma_pages().

pmd_trans_huge_lock() succeeds for such PMDs (pmd_is_huge() returns true for any non-present, non-none huge PMD), so several MM walk callbacks that used to assume present THP or migration entry are now reachable with a device-private PMD. The results range from a VM_BUG_ON() firing on debug kernels, to an oops on a bogus vmemmap dereference, to silently isolating an unrelated live folio from LRU in the aliasing case.

This patch (of 3):

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.

Affected

• Linux >= 368076f52ebeecd33e10a9f80905d7508b6b6149 < 3d927093b2add7ced588a926c3dc5d7a83f18596
• Linux >= 368076f52ebeecd33e10a9f80905d7508b6b6149 < 2858b4027f491e6fdb8ee2d8923798b619bf2791
• Linux 6.19

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · affected: Red Hat Enterprise Linux 6 · no fix planned: Red Hat Enterprise Linux 6 · updated 2026-09-21 · vex