Add support for the new vm_uffd_ops API for shmem.  Note that this only
introduces the support, the API is not yet used by core mm.

It only needs a separate uffd_get_folio() definition but that's oneliner.

Due to the limitation of the current vm_uffd_ops on MISSING mode support,
the shmem UFFDIO_COPY/ZEROPAGE process are still hard-coded in mm/.

Cc: Hugh Dickins <hughd@google.com>
Acked-by: Mike Rapoport <rppt@kernel.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/shmem.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 4855eee227310..e7b44efbfddf2 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3148,6 +3148,13 @@ static inline struct inode *shmem_get_inode(struct mnt_idmap *idmap,
 #endif /* CONFIG_TMPFS_QUOTA */
 
 #ifdef CONFIG_USERFAULTFD
+
+static int shmem_uffd_get_folio(struct inode *inode, pgoff_t pgoff,
+				struct folio **folio)
+{
+	return shmem_get_folio(inode, pgoff, 0, folio, SGP_NOALLOC);
+}
+
 int shmem_mfill_atomic_pte(pmd_t *dst_pmd,
 			   struct vm_area_struct *dst_vma,
 			   unsigned long dst_addr,
@@ -5191,6 +5198,18 @@ static int shmem_error_remove_folio(struct address_space *mapping,
 	return 0;
 }
 
+#ifdef CONFIG_USERFAULTFD
+static const struct vm_uffd_ops shmem_uffd_ops = {
+	.uffd_features	=	__VM_UFFD_FLAGS,
+	.uffd_ioctls	=	BIT(_UFFDIO_COPY) |
+				BIT(_UFFDIO_ZEROPAGE) |
+				BIT(_UFFDIO_WRITEPROTECT) |
+				BIT(_UFFDIO_CONTINUE) |
+				BIT(_UFFDIO_POISON),
+	.uffd_get_folio	=	shmem_uffd_get_folio,
+};
+#endif
+
 static const struct address_space_operations shmem_aops = {
 	.dirty_folio	= noop_dirty_folio,
 #ifdef CONFIG_TMPFS
@@ -5293,6 +5312,9 @@ static const struct vm_operations_struct shmem_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.userfaultfd_ops = &shmem_uffd_ops,
+#endif
 };
 
 static const struct vm_operations_struct shmem_anon_vm_ops = {
@@ -5302,6 +5324,9 @@ static const struct vm_operations_struct shmem_anon_vm_ops = {
 	.set_policy     = shmem_set_policy,
 	.get_policy     = shmem_get_policy,
 #endif
+#ifdef CONFIG_USERFAULTFD
+	.userfaultfd_ops = &shmem_uffd_ops,
+#endif
 };
 
 int shmem_init_fs_context(struct fs_context *fc)
-- 
2.50.1