Re: [PATCH v4 1/6] fs: add write-stream management ioctls

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Jul 17, 2026 at 06:25:33PM +0530, Kanchan Joshi wrote:
> From: Anuj Gupta <anuj20.g@samsung.com>
> 
> Wire up the userspace interface for write stream management via four
> ioctls:
> 
>   FS_IOC_WRITE_STREAM_GET_MAX  query the max streams supported
>   FS_IOC_WRITE_STREAM_OPEN     open a stream id, returns a stream fd
>   FS_IOC_WRITE_STREAM_SET      attach the stream fd to an open file
>   FS_IOC_WRITE_STREAM_GET      query the stream id value set on a file
> 
> Application should query the available streams by using
> FS_IOC_WRITE_STREAM_GET_MAX. If returned value is N, valid stream id for
> the file are 1 to N.
> 
> Application calls FS_IOC_WRITE_STREAM_OPEN to get fd for a stream-id.
> By default, kernel picks an available stream-id and returns the fd for
> it. The flag FS_WRITE_STREAM_OPEN_EXACT can be used to request a
> specific stream_id. This is useful if application cares about keeping a
> stable stream-id-to-spatial-isolation-bucket mapping across restarts.

Hmm.  If FS_IOC_WRITE_STREAM_OPEN returns an fd that represents an open
stream id, then why does STREAM_SET below take a pointer to a signed
s32?  And why does STREAM_GET take a pointer to a u32?

IOWs it'd be much easier to distinguish these things if the ioctls took
pointers to structs instead of u32/s32 pointers directly.

Also it's a little weird that STREAM_SET associates an open file with a
stream fd, but STREAM_GET returns the stream *id* (not the fd)
associated with an open file.

Can you extract the stream id from the fd that FS_IOC_WRITE_STREAM_OPEN
returns?

--D

> Suggested-by: Christoph Hellwig <hch@lst.de>
> Co-developed-by: Kanchan Joshi <joshi.k@samsung.com>
> Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> ---
>  include/uapi/linux/fs.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index bd87262f2e34..a756d27bdf84 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -345,6 +345,22 @@ struct file_attr {
>  /* Get logical block metadata capability details */
>  #define FS_IOC_GETLBMD_CAP		_IOWR(0x15, 2, struct logical_block_metadata_cap)
>  
> +struct fs_write_stream_open {
> +	__u32		stream_id;	/* IN: requested id if OPEN_EXACT set; OUT: assigned id */
> +	__u32		flags;		/* IN: FS_WRITE_STREAM_OPEN_* */
> +};
> +
> +/*
> + * Flag to ask for specific stream_id, otherwise kernel can return any.
> + * Fails with -EBUSY if that stream_id is already held
> + */
> +#define FS_WRITE_STREAM_OPEN_EXACT	(1 << 0)
> +
> +#define FS_IOC_WRITE_STREAM_GET_MAX	_IOR('f', 135, __u32)
> +#define FS_IOC_WRITE_STREAM_OPEN	_IOWR('f', 136, struct fs_write_stream_open)
> +#define FS_IOC_WRITE_STREAM_SET		_IOW('f', 137, __s32)
> +#define FS_IOC_WRITE_STREAM_GET		_IOR('f', 138, __u32)
> +
>  /*
>   * Inode flags (FS_IOC_GETFLAGS / FS_IOC_SETFLAGS)
>   *
> -- 
> 2.25.1
> 
>