[PATCH v2 02/31] exportfs: add new EXPORT_OP_STABLE_HANDLES flag

[Amir Goldstein]

On Mon, Jan 19, 2026 at 5:27 PM Jeff Layton <jlayton at kernel.org> wrote:
>
> At one time, nfsd could take the presence of struct export_operations to
> be an indicator that a filesystem was exportable via NFS. Since then, a
> lot of filesystems have grown export operations in order to provide
> filehandle support. Some of those (e.g. kernfs, pidfs, and nsfs) are not
> suitable for export via NFS since they lack filehandles that are
> stable across reboot.
>
> Add a new EXPORT_OP_STABLE_HANDLES flag that indicates that the
> filesystem supports perisistent filehandles,

persistent still here?
"...are stable across the lifetime of a file"?

> a requirement for nfs
> export. While in there, switch to the BIT() macro for defining these
> flags.

Maybe you want to move that cleanup to patch 1 along with the
export.rst sync? not a must.

>
> For now, the flag is not checked anywhere. That will come later after
> we've added it to the existing filesystems that need to remain
> exportable.
>
> Reviewed-by: Jan Kara <jack at suse.cz>
> Signed-off-by: Jeff Layton <jlayton at kernel.org>
> ---
>  Documentation/filesystems/nfs/exporting.rst |  7 +++++++
>  include/linux/exportfs.h                    | 16 +++++++++-------
>  2 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/filesystems/nfs/exporting.rst b/Documentation/filesystems/nfs/exporting.rst
> index 0583a0516b1e3a3e6a10af95ff88506cf02f7df4..0c29ee44e3484cef84d2d3d47819acf172d275a3 100644
> --- a/Documentation/filesystems/nfs/exporting.rst
> +++ b/Documentation/filesystems/nfs/exporting.rst
> @@ -244,3 +244,10 @@ following flags are defined:
>      nfsd. A case in point is reexport of NFS itself, which can't be done
>      safely without coordinating the grace period handling. Other clustered
>      and networked filesystems can be problematic here as well.
> +
> +  EXPORT_OP_STABLE_HANDLES - This filesystem provides filehandles that are
> +    stable across the lifetime of a file. This is a hard requirement for export
> +    via nfsd. Any filesystem that is eligible to be exported via nfsd must
> +    indicate this guarantee by setting this flag. Most disk-based filesystems
> +    can do this naturally. Pseudofilesystems that are for local reporting and
> +    control (e.g. kernfs, pidfs, nsfs) usually can't support this.
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index f0cf2714ec52dd942b8f1c455a25702bd7e412b3..c4e0f083290e7e341342cf0b45b58fddda3af65e 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -3,6 +3,7 @@
>  #define LINUX_EXPORTFS_H 1
>
>  #include <linux/types.h>
> +#include <linux/bits.h>
>  #include <linux/path.h>
>
>  struct dentry;
> @@ -277,15 +278,16 @@ struct export_operations {
>                              int nr_iomaps, struct iattr *iattr);
>         int (*permission)(struct handle_to_path_ctx *ctx, unsigned int oflags);
>         struct file * (*open)(const struct path *path, unsigned int oflags);
> -#define        EXPORT_OP_NOWCC                 (0x1) /* don't collect v3 wcc data */
> -#define        EXPORT_OP_NOSUBTREECHK          (0x2) /* no subtree checking */
> -#define        EXPORT_OP_CLOSE_BEFORE_UNLINK   (0x4) /* close files before unlink */
> -#define EXPORT_OP_REMOTE_FS            (0x8) /* Filesystem is remote */
> -#define EXPORT_OP_NOATOMIC_ATTR                (0x10) /* Filesystem cannot supply
> +#define EXPORT_OP_NOWCC                        BIT(0) /* don't collect v3 wcc data */
> +#define EXPORT_OP_NOSUBTREECHK         BIT(1) /* no subtree checking */
> +#define EXPORT_OP_CLOSE_BEFORE_UNLINK  BIT(2) /* close files before unlink */
> +#define EXPORT_OP_REMOTE_FS            BIT(3) /* Filesystem is remote */
> +#define EXPORT_OP_NOATOMIC_ATTR                BIT(4) /* Filesystem cannot supply
>                                                   atomic attribute updates
>                                                 */
> -#define EXPORT_OP_FLUSH_ON_CLOSE       (0x20) /* fs flushes file data on close */
> -#define EXPORT_OP_NOLOCKS              (0x40) /* no file locking support */
> +#define EXPORT_OP_FLUSH_ON_CLOSE       BIT(5) /* fs flushes file data on close */
> +#define EXPORT_OP_NOLOCKS              BIT(6) /* no file locking support */
> +#define EXPORT_OP_STABLE_HANDLES       BIT(7) /* fhs are stable across reboot */
>         unsigned long   flags;
>  };
>
>
> --
> 2.52.0
>