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

Peter Xu
2024-08-09T16:09:36Z

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@redhat.com>
Signed-off-by: Peter Xu <peterx@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