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

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 10-21 12:14, Pratyush Yadav wrote:
> 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>

Thank you, I am going to send out a new version soon.

Pasha

> 
> > +	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