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

On Fri, Jun 27, 2025 at 11:46:53AM -0400, Peter Xu wrote:
> 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>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  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
> 
> 

-- 
Sincerely yours,
Mike.