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

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.