Re: [PATCH v2 1/5] fs: add generic write-stream management ioctl

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

On Tue, Mar 10, 2026 at 11:25:25PM +0530, Kanchan Joshi wrote:
> On 3/9/2026 10:03 PM, Darrick J. Wong wrote:
> >> +struct fs_write_stream {
> >> +	__u32		op_flags;	/* IN: operation flags */
> >> +	__u32		stream_id;	/* IN/OUT:  stream value to assign/guery */
> >> +	__u32		max_streams;	/* OUT: max streams values supported */
> >> +	__u32		rsvd;
> >> +};
> > This isn't an very cohesive interface -- GET_MAX probably only needs
> > op_flags and max_streams, right?  And GET/SET only use op_flags and
> > stream_id, right?
> 
> Yeah, right. That's the trade-off with swiss army knife type ioctl which 
> uses op_flags to decide what it should do. Apart from keeping a single 
> ioctl I was thinking a bit about extensibility (for anything new we may 
> be able to do a new op_flags with some rsvd or union) too. But if you 
> feel strong about this, I can take 3 ioctl route?

struct fs_write_stream {
	__u32		op_flags;
	union {
		__u32	stream_id;
		__u32	max_ids;
	};
	__u64		reserved;
};

perhaps?  You might want to look into whether or not we're allowed to
have anonymous unions in UAPI headers.  We all ❤️ C11, right?

--D

> >> +#define FS_WRITE_STREAM_OP_GET_MAX		(1 << 0)
> >> +#define FS_WRITE_STREAM_OP_GET			(1 << 1)
> >> +#define FS_WRITE_STREAM_OP_SET			(1 << 2)
> >> +
> >> +#define FS_IOC_WRITE_STREAM		_IOWR('f', 43, struct fs_write_stream)
> > EXT4_IOC_CHECKPOINT already took 'f' / 43.  I/think/ there's no problem
> > because its argument is a u32 and ioctl definitions incorporate the
> > lower bits of of the argument size but you might want to be careful
> > anyway.
> 
> Indeed, thanks!
>