mm/shmem: persist uffd-wp bit across zapping for file-backed

File-backed memory is prone to being unmapped at any time.  It means all
information in the pte will be dropped, including the uffd-wp flag.

To persist the uffd-wp flag, we'll use the pte markers.  This patch
teaches the zap code to understand uffd-wp and know when to keep or drop
the uffd-wp bit.

Add a new flag ZAP_FLAG_DROP_MARKER and set it in zap_details when we
don't want to persist such an information, for example, when destroying
the whole vma, or punching a hole in a shmem file.  For the rest cases we
should never drop the uffd-wp bit, or the wr-protect information will get
lost.

The new ZAP_FLAG_DROP_MARKER needs to be put into mm.h rather than
memory.c because it'll be further referenced in hugetlb files later.

Link: https://lkml.kernel.org/r/20220405014847.14295-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/mm/rmap.c b/mm/rmap.c
index 72464378e1a6..94d6b24a1ac2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -73,6 +73,7 @@
 #include <linux/page_idle.h>
 #include <linux/memremap.h>
 #include <linux/userfaultfd_k.h>
+#include <linux/mm_inline.h>
 
 #include <asm/tlbflush.h>
 
@@ -1585,6 +1586,13 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 			pteval = ptep_clear_flush(vma, address, pvmw.pte);
 		}
 
+		/*
+		 * Now the pte is cleared. If this pte was uffd-wp armed,
+		 * we may want to replace a none pte with a marker pte if
+		 * it's file-backed, so we don't lose the tracking info.
+		 */
+		pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
+
 		/* Set the dirty flag on the folio now the pte is gone. */
 		if (pte_dirty(pteval))
 			folio_mark_dirty(folio);