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

[Christoph Hellwig <hch@lst.de>]

On Thu, Jul 30, 2026 at 08:22:51PM +0530, Kanchan Joshi wrote:
> Christoph's idea was to use FD for a write-stream so that it remains 
> exclusive to the application and unrelated applications don't collide.
> 
> So, in this version, write-stream ids are implemented as independent 
> resource:
> - To set a file's write-stream ID to X, the app must first obtain the fd 
> for stream X.
> - That stream/fd will be unavailable if some other application has 
> already opened it.
> - If applications gets the stream fd, it can use that to set stream id 
> for multiple files.
> - It can close the stream-fd and that does not change anything for the 
> file resource (i.e, its inode continue to carry the stream-id value that 
> was set).
> 
> Christoph - does this match?

Almost.  I didn't really think of your last point there.  If the
inodes keeps using it when it is dropped that breaks the model a
bit, but revoking it might make things a bit ugly and slow.  Urgg.

> Here are the revised names (suggestions?):
> 
> 1. FS_IOC_QUERY_MAX_WRITE_STREAM_IDS		_IOR('f', 135, __u32)
> 
> Returns (in __u32) number of supported stream-ids.
> 
> 2. FS_IOC_OPEN_WRITE_STREAM_ID			_IOWR('f', 136, struct 
> fs_write_stream_open)
> 
> Open the specific write-stream id (or any available one via a flag) and 
> return its FD. This gives right to use that stream-id.

maybe s/OPEN/ALLOC(ATA)/ ?

> And the following manage setting/clearing/querying write-stream on the file:
> 
> 3. FS_IOC_SET_FILE_WRITE_STREAM_BY_FD		_IOW('f', 137, __s32)
> 
> Sets a file's write-stream value (in inode) to whatever the stream fd 
> (passed as __s32) carries. So one never sets scalar values into a file 
> directly.

I'd drop the _BY_FD.

> 
> 4. FS_IOC_QUERY_FILE_WRITE_STREAM_ID		_IOR('f', 138, __u32)
> 
> Queries the write-stream value (__u32) currently assigned to the file 
> (its inode).

I'm not sure exposing the actual ID to the user space application is a
that good idea, same for passing in the wanted ID when allocating.
That leaks a lot of internal details.