On Fri, Sep 12, 2025 at 5:27 PM Thomas Bertschinger
<tahbertschinger@xxxxxxxxx> wrote:
>
> This defines a new flag FILEID_CACHED that the VFS can set in the
> handle_type field of struct file_handle to request that the FS
> implementations of fh_to_{dentry,parent}() only complete if they can
> satisfy the request with cached data.
>
> Because not every FS implementation will recognize this new flag, those
> that do recognize the flag can indicate their support using a new
> export flag, EXPORT_OP_NONBLOCK.
>
> If FILEID_CACHED is set in a file handle, but the filesystem does not
> set EXPORT_OP_NONBLOCK, then the VFS will return -EAGAIN without
> attempting to call into the filesystem code.
>
> exportfs_decode_fh_raw() is updated to respect the new flag by returning
> -EAGAIN when it would need to do an operation that may not be possible
> with only cached data.
>
> Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Thomas Bertschinger <tahbertschinger@xxxxxxxxx>
> ---
> I didn't apply Amir's Reviewed-by for this patch because I added the
> Documenation section, which was not reviewed in v2.
Documentation looks good.
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Thanks,
Amir.
>
> Documentation/filesystems/nfs/exporting.rst | 6 ++++++
> fs/exportfs/expfs.c | 12 ++++++++++++
> fs/fhandle.c | 2 ++
> include/linux/exportfs.h | 5 +++++
> 4 files changed, 25 insertions(+)
>
> diff --git a/Documentation/filesystems/nfs/exporting.rst b/Documentation/filesystems/nfs/exporting.rst
> index de64d2d002a2..70f46eaeb0d4 100644
> --- a/Documentation/filesystems/nfs/exporting.rst
> +++ b/Documentation/filesystems/nfs/exporting.rst
> @@ -238,3 +238,9 @@ following flags are defined:
> all of an inode's dirty data on last close. Exports that behave this
> way should set EXPORT_OP_FLUSH_ON_CLOSE so that NFSD knows to skip
> waiting for writeback when closing such files.
> +
> + EXPORT_OP_NONBLOCK - FS supports fh_to_{dentry,parent}() using cached data
> + When performing open_by_handle_at(2) using io_uring, it is useful to
> + complete the file open using only cached data when possible, otherwise
> + failing with -EAGAIN. This flag indicates that the filesystem supports this
> + mode of operation.
> diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
> index 949ce6ef6c4e..e2cfdd9d6392 100644
> --- a/fs/exportfs/expfs.c
> +++ b/fs/exportfs/expfs.c
> @@ -441,6 +441,7 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
> void *context)
> {
> const struct export_operations *nop = mnt->mnt_sb->s_export_op;
> + bool decode_cached = fileid_type & FILEID_CACHED;
> struct dentry *result, *alias;
> char nbuf[NAME_MAX+1];
> int err;
> @@ -453,6 +454,10 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
> */
> if (!exportfs_can_decode_fh(nop))
> return ERR_PTR(-ESTALE);
> +
> + if (decode_cached && !(nop->flags & EXPORT_OP_NONBLOCK))
> + return ERR_PTR(-EAGAIN);
> +
> result = nop->fh_to_dentry(mnt->mnt_sb, fid, fh_len, fileid_type);
> if (IS_ERR_OR_NULL(result))
> return result;
> @@ -481,6 +486,10 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
> * filesystem root.
> */
> if (result->d_flags & DCACHE_DISCONNECTED) {
> + err = -EAGAIN;
> + if (decode_cached)
> + goto err_result;
> +
> err = reconnect_path(mnt, result, nbuf);
> if (err)
> goto err_result;
> @@ -526,6 +535,9 @@ exportfs_decode_fh_raw(struct vfsmount *mnt, struct fid *fid, int fh_len,
> err = PTR_ERR(target_dir);
> if (IS_ERR(target_dir))
> goto err_result;
> + err = -EAGAIN;
> + if (decode_cached && (target_dir->d_flags & DCACHE_DISCONNECTED))
> + goto err_result;
>
> /*
> * And as usual we need to make sure the parent directory is
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 2dc669aeb520..509ff8983f94 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -273,6 +273,8 @@ static int do_handle_to_path(struct file_handle *handle, struct path *path,
> if (IS_ERR_OR_NULL(dentry)) {
> if (dentry == ERR_PTR(-ENOMEM))
> return -ENOMEM;
> + if (dentry == ERR_PTR(-EAGAIN))
> + return -EAGAIN;
> return -ESTALE;
> }
> path->dentry = dentry;
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index 30a9791d88e0..8238b6f67956 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -199,6 +199,8 @@ struct handle_to_path_ctx {
> #define FILEID_FS_FLAGS_MASK 0xff00
> #define FILEID_FS_FLAGS(flags) ((flags) & FILEID_FS_FLAGS_MASK)
>
> +#define FILEID_CACHED 0x100 /* Use only cached data when decoding handle */
> +
> /* User flags: */
> #define FILEID_USER_FLAGS_MASK 0xffff0000
> #define FILEID_USER_FLAGS(type) ((type) & FILEID_USER_FLAGS_MASK)
> @@ -303,6 +305,9 @@ struct export_operations {
> */
> #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_NONBLOCK (0x80) /* Filesystem supports non-
> + blocking fh_to_dentry()
> + */
> unsigned long flags;
> };
>
> --
> 2.51.0
>
