[PATCH 06/19] mm/pagewalk: Check pfnmap early for folio_walk_start()

[Peter Xu]

Pfnmaps can always be identified with special bits in the ptes/pmds/puds.
However that's unnecessary if the vma is stable, and when it's mapped under
VM_PFNMAP | VM_IO.

Instead of adding similar checks in all the levels for huge pfnmaps, let
folio_walk_start() fail even earlier for these mappings.  It's also
something gup-slow already does, so make them match.

Cc: David Hildenbrand <david at redhat.com>
Signed-off-by: Peter Xu <peterx at redhat.com>
---
 mm/pagewalk.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index cd79fb3b89e5..fd3965efe773 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -727,6 +727,11 @@ struct folio *folio_walk_start(struct folio_walk *fw,
 	p4d_t *p4dp;
 
 	mmap_assert_locked(vma->vm_mm);
+
+	/* It has no folio backing the mappings at all.. */
+	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+		return NULL;
+
 	vma_pgtable_walk_begin(vma);
 
 	if (WARN_ON_ONCE(addr < vma->vm_start || addr >= vma->vm_end))
-- 
2.45.0