mm/pagewalk: check pfnmap for folio_walk_start()

Teach folio_walk_start() to recognize special pmd/pud mappings, and fail
them properly as it means there's no folio backing them.

[[email protected]: remove some stale comments, per David]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Alex Williamson <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Gavin Shan <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Niklas Schnelle <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/memory.c

@@ -672,11 +672,10 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
 {
 	unsigned long pfn = pmd_pfn(pmd);
 
-	/*
-	 * There is no pmd_special() but there may be special pmds, e.g.
-	 * in a direct-access (dax) mapping, so let's just replicate the
-	 * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
-	 */
+	/* Currently it's only used for huge pfnmaps */
+	if (unlikely(pmd_special(pmd)))
+		return NULL;
+
 	if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
 		if (vma->vm_flags & VM_MIXEDMAP) {
 			if (!pfn_valid(pfn))

mm/pagewalk.c

@@ -753,7 +753,7 @@ struct folio *folio_walk_start(struct folio_walk *fw,
 		fw->pudp = pudp;
 		fw->pud = pud;
 
-		if (!pud_present(pud) || pud_devmap(pud)) {
+		if (!pud_present(pud) || pud_devmap(pud) || pud_special(pud)) {
 			spin_unlock(ptl);
 			goto not_found;
 		} else if (!pud_leaf(pud)) {