Re: [PATCH v2 1/2] liveupdate: Add LIVEUPDATE_SESSION_FREEZE ioctl to freeze sessions early

[Pratyush Yadav <pratyush@kernel.org>]

On Tue, Oct 20 2026, Pasha Tatashin wrote:

> Currently, LUO sessions are frozen automatically only inside the
> reboot(LINUX_REBOOT_CMD_KEXEC) pathway, where luo_session_serialize()
> iterates through all outgoing sessions under global write locks and
> freezes them serially during shutdown. Allowing userspace to decide when
> to freeze a session once it is done with it (and to freeze multiple
> sessions in parallel, since luo_session_freeze() only acquires the
> per-session mutex) avoids postponing this work to kexec shutdown and
> reduces blackout time.
>
> Introduce LIVEUPDATE_SESSION_FREEZE on session file descriptors to freeze
> a session early. The ioctl runs .freeze() across all preserved files in the
> session and serializes their metadata. Once frozen, no additional files can
> be added to the session, and subsequent freeze calls return -EBUSY. Freeze
> is only valid on outgoing sessions and returns -EINVAL on incoming ones.
>
> During kexec, sessions that were already frozen early are skipped, while
> sessions that did not invoke this ioctl auto-freeze as before. If an
> early-frozen session is released before kexec, its files are rolled back
> via .unfreeze() before being unpreserved.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
[...]
> @@ -218,18 +219,33 @@ static int luo_session_finish_one(struct luo_session *session)
>  	return luo_file_finish(&session->file_set);
>  }
>  
> -static void luo_session_unfreeze_one(struct luo_session *session,
> -				     struct luo_session_ser *ser)
> +static void luo_session_unserialize_one(struct luo_session *session,
> +					struct luo_session_ser *ser)
>  {
>  	guard(mutex)(&session->mutex);
> -	luo_file_unfreeze(&session->file_set, &ser->file_set_ser);
> +	if (!session->frozen)
> +		luo_file_unfreeze(&session->file_set);

"Wait, this looks wrong. If session is not frozen then unfreeze it?
Should be the other way round, right?"

This was my first reaction until I realized that you only unfreeze
_automatically_ frozen sessions. User-frozen sessions stay frozen. Which
makes sense, but is not obvious at all.

Perhaps rename this to session->user_frozen and add a comment here?

With that fixed, feel free to add

Reviewed-by: Pratyush Yadav <pratyush@kernel.org>

> +	memset(ser, 0, sizeof(*ser));
>  }
>  
> -static int luo_session_freeze_one(struct luo_session *session,
> -				  struct luo_session_ser *ser)
> +static int luo_session_serialize_one(struct luo_session *session,
> +				     struct luo_session_ser *ser)
>  {
> +	int err;
> +
>  	guard(mutex)(&session->mutex);
> -	return luo_file_freeze(&session->file_set, &ser->file_set_ser);
> +	if (!session->frozen) {
> +		err = luo_file_freeze(&session->file_set);
> +		if (err)
> +			return err;
> +	}
> +
> +	strscpy(ser->name, session->name, sizeof(ser->name));
> +	ser->file_set_ser.count = session->file_set.count;
> +	ser->file_set_ser.files =
> +		kho_block_set_head_pa(&session->file_set.block_set);
> +
> +	return 0;
>  }
>  
>  static int luo_session_release(struct inode *inodep, struct file *filep)
[...]
> @@ -267,6 +286,9 @@ static int luo_session_preserve_fd(struct luo_session *session,
>  	int err;
>  
>  	guard(mutex)(&session->mutex);
> +	if (session->frozen)
> +		return -EBUSY;
> +
>  	err = luo_preserve_file(&session->file_set, argp->token, argp->fd);
>  	if (err)
>  		return err;
> @@ -340,11 +362,40 @@ static int luo_session_get_name(struct luo_session *session,
>  	return luo_ucmd_respond(ucmd, sizeof(*argp));
>  }
>  
> +static int luo_session_freeze(struct luo_session *session,
> +			      struct luo_ucmd *ucmd)
> +{
> +	struct liveupdate_session_freeze *argp = ucmd->cmd;
> +	int err;
> +
> +	if (argp->reserved)
> +		return -EINVAL;
> +
> +	guard(mutex)(&session->mutex);
> +	if (session->frozen)
> +		return -EBUSY;
> +
> +	err = luo_file_freeze(&session->file_set);
> +	if (err)
> +		return err;
> +
> +	err = luo_ucmd_respond(ucmd, sizeof(*argp));
> +	if (err) {
> +		luo_file_unfreeze(&session->file_set);
> +		return err;
> +	}
> +
> +	session->frozen = true;
> +
> +	return 0;
> +}
> +
>  union ucmd_buffer {
>  	struct liveupdate_session_finish finish;
>  	struct liveupdate_session_preserve_fd preserve;
>  	struct liveupdate_session_retrieve_fd retrieve;
>  	struct liveupdate_session_get_name get_name;
> +	struct liveupdate_session_freeze freeze;
>  };
>  
>  /* Type of sessions the ioctl applies to. */
[...]

-- 
Regards,
Pratyush Yadav