[PATCH v2 2/4] mm/shmem: Support vm_uffd_ops API

Peter Xu
2025-06-27T15:47:07Z

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.

Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
mm/shmem.c | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2b19965d27df..9a8b8dd4709b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3151,6 +3151,13 @@ 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,
@@ -5194,6 +5201,19 @@ 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
@@ -5296,6 +5316,9 @@ 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 = {
@@ -5305,6 +5328,9 @@ 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)
--
2.49.0