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

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

[Darrick J. Wong]

[cc linux-api because this is certainly an API definition]

On Mon, Mar 09, 2026 at 10:59:40AM +0530, Kanchan Joshi wrote:
> Wire up the userspace interface for write stream management via a new
> vfs ioctl 'FS_IOC_WRITE_STEAM'.
> Application communictes the intended operation using the 'op_flags'
> field of the passed 'struct fs_write_stream'.
> Valid flags are:
> FS_WRITE_STREAM_OP_GET_MAX: Returns the number of available streams.
> FS_WRITE_STREAM_OP_SET: Assign a specific stream value to the file.
> FS_WRITE_STREAM_OP_GET: Query what stream value is set on the file.
> 
> Application should query the available streams by using
> FS_WRITE_STREAM_OP_GET_MAX first.
> If returned value is N, valid stream values for the file are 0 to N.
> Stream value 0 implies that no stream is set on the file.
> Setting a larger value than available streams is rejected.
> 
> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> ---
>  include/uapi/linux/fs.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 70b2b661f42c..4d0805b52949 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -338,6 +338,18 @@ 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 {
> +	__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?

> +#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.

--D

>  /*
>   * Inode flags (FS_IOC_GETFLAGS / FS_IOC_SETFLAGS)
>   *
> -- 
> 2.25.1
> 
> 

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

[Kanchan Joshi]

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?

>> +#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!

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

[Darrick J. Wong]

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!
>