mm/shmem: support vm_uffd_ops API

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.

Due to the tailored uffd_copy() API, shmem is extremely easy to support it
by reusing the existing mfill function.

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

Link: https://lkml.kernel.org/r/20250627154655.2085903-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: James Houghton <jthoughton@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: xzpeter

mm/shmem.c

@@ -3161,6 +3161,13 @@ 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,
@@ -5204,6 +5211,19 @@ static int shmem_error_remove_folio(struct address_space *mapping,
 	return 0;
 }
 
+#ifdef CONFIG_USERFAULTFD
+static const 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,
+	.uffd_copy	=	shmem_mfill_atomic_pte,
+};
+#endif
+
 static const struct address_space_operations shmem_aops = {
 	.dirty_folio	= noop_dirty_folio,
 #ifdef CONFIG_TMPFS
@@ -5306,6 +5326,9 @@ 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 = {
@@ -5315,6 +5338,9 @@ 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)