[PATCH 1/1] mm/huge_memory: fix pgtable withdrawal for huge zero PMDs

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.dev>

has_deposited_pgtable() uses !vma_is_dax() to decide whether a huge zero
PMD has a deposited PTE page table. That also accepts raw PFN mappings
of huge_zero_pfn, although vmf_insert_pfn_pmd() does not deposit a page
table on x86.

Zapping such a mapping would call pgtable_trans_huge_withdraw() without
a corresponding deposit. With pmd_huge_pte(mm, pmd) == NULL, that causes
a NULL pointer dereference.

Use vma_is_anonymous() for the huge zero PMD check. This matches how PTE
page tables are allocated, deposited and moved.

- For anonymous page faults that install a huge zero PMD,
  do_huge_pmd_anonymous_page() allocates a PTE page table and
  set_huge_zero_folio() deposits it before installing the PMD.

- On fork, copy_huge_pmd() allocates and deposits a PTE page table when
  copying a huge zero PMD into an anonymous VMA.

- Raw PFN mappings use vmf_insert_pfn_pmd(), and DAX file holes use
  vmf_insert_folio_pmd() to map the huge zero folio. Both use insert_pmd(),
  which deposits a PTE page table only when arch_needs_pgtable_deposit()
  requires it.

- Moving an anonymous huge PMD preserves its deposited PTE page table.
  move_huge_pmd() transfers the deposit when necessary. For UFFD MOVE,
  both VMAs must be anonymous, and move_pages_huge_pmd() transfers the
  deposit as well.

Keep arch_needs_pgtable_deposit() first so architectures that require a
deposited PTE page table still return true regardless of the VMA type.

Commit d80a9cb1a64a ("mm/huge_memory: add and use
normal_or_softleaf_folio_pmd()") removed the vma_is_special_huge() check
in zap_huge_pmd(). That check skipped the huge zero PMD deposit test for
non-DAX VM_PFNMAP and VM_MIXEDMAP mappings. Removing it exposed these
mappings to the incorrect !vma_is_dax() test.

Fixes: d80a9cb1a64a ("mm/huge_memory: add and use normal_or_softleaf_folio_pmd()")
Cc: stable@vger.kernel.org
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 mm/huge_memory.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 6895b38e4704..0ed997a97416 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2529,11 +2529,11 @@ static bool has_deposited_pgtable(struct vm_area_struct *vma, pmd_t pmdval,
 		return true;
 
 	/*
-	 * Huge zero always deposited except for DAX which handles itself, see
-	 * set_huge_zero_folio().
+	 * Huge zero PMDs have a deposited page table only for anonymous VMAs,
+	 * see set_huge_zero_folio().
 	 */
 	if (is_huge_zero_pmd(pmdval))
-		return !vma_is_dax(vma);
+		return vma_is_anonymous(vma);
 
 	/*
 	 * Otherwise, only anonymous folios are deposited, see
-- 
2.39.3 (Apple Git-146)