On 09.08.24 18:08, Peter Xu wrote:
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@xxxxxxxxxx>
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
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;
+
That is in general not what we want, and we still have some places that
wrongly hard-code that behavior.
In a MAP_PRIVATE mapping you might have anon pages that we can happily walk.
vm_normal_page() / vm_normal_page_pmd() [and as commented as a TODO,
vm_normal_page_pud()] should be able to identify PFN maps and reject
them, no?
--
Cheers,
David / dhildenb
