On Wed, Oct 29, 2025 at 4:44 PM David Matlack [off-list ref] wrote:

On Wed, Oct 29, 2025 at 1:13 PM Pasha Tatashin
[off-list ref] wrote:

quoted

On Wed, Oct 29, 2025 at 3:07 PM Pratyush Yadav [off-list ref] wrote:

quoted

Also, I think the model we should have is to only allow new sessions in
normal state. Currently luo_session_create() allows creating a new
session in updated state. This would end up mixing sessions from a
previous boot and sessions from current boot. I don't really see a
reason for that and I think the userspace should first call finish
before starting new serialization. Keeps things simpler.

It does. However, yesterday Jason Gunthorpe suggested that we simplify
the uapi, at least for the initial landing, by removing the state
machine during boot and allowing new sessions to be created at any
time. This would also mean separating the incoming and outgoing
sessions and removing the ioctl() call used to bring the machine into
a normal state; instead, only individual sessions could be brought
into a 'normal' state.

Simplified uAPI Proposal
The simplest uAPI would look like this:
IOCTLs on /dev/liveupdate (to create and retrieve session FDs):
LIVEUPDATE_IOCTL_CREATE_SESSION
LIVEUPDATE_IOCTL_RETRIEVE_SESSION

IOCTLs on session FDs:
LIVEUPDATE_CMD_SESSION_PRESERVE_FD
LIVEUPDATE_CMD_SESSION_RETRIEVE_FD
LIVEUPDATE_CMD_SESSION_FINISH

Should we drop LIVEUPDATE_CMD_SESSION_FINISH and do this work in
close(session_fd)? close() can return an error.

I think this cleans up a few parts of the uAPI:

 - One less ioctl.
 - The only way to get an outgoing session would be through
LIVEUPDATE_IOCTL_CREATE_SESSION. The kernel does not have to deal with
an empty incoming session "becoming" an outgoing session (as described
below).
 - The kernel can properly leak the session and its resources by
refusing to close the session file.

I was considering this. But, in AFAIK even if close() fails, the FD is
still closed, therefore, I am not aware of any existing api that
relies on close() to fail. The finish or (set event if we decide to
expands events in the future) should be a separate ioctl() and close()
should release FD unconditionally as it still would do even if return
failure from release()