linux-kernel - RE: [PATCH v2 4/4] hv_utils: Add the support of hibernation

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Message-ID: <HK0P153MB0148D8719B52A34BDB6C5836BF0F0@HK0P153MB0148.APCP153.PROD.OUTLOOK.COM>
Date:   Thu, 23 Jan 2020 08:11:32 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        Sasha Levin <Alexander.Levin@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 4/4] hv_utils: Add the support of hibernation

> From: Michael Kelley <mikelley@...rosoft.com>
> Sent: Wednesday, January 22, 2020 10:28 AM
> ...
> > +int hv_fcopy_pre_suspend(void)
> > +{
> > +	struct vmbus_channel *channel = fcopy_transaction.recv_channel;
> > +	struct hv_fcopy_hdr *fcopy_msg;
> > +
> > +	tasklet_disable(&channel->callback_event);
> > + ...
> > +	fcopy_msg = kzalloc(sizeof(*fcopy_msg), GFP_KERNEL);
> > +	if (!fcopy_msg)
> > +		goto err;
> > +
> > +	fcopy_msg->operation = CANCEL_FCOPY;
> > +
> > +	hv_fcopy_cancel_work();
> > +
> > +	/* We don't care about the return value. */
> > +	hvutil_transport_send(hvt, fcopy_msg, sizeof(*fcopy_msg), NULL);
> > +
> > +	kfree(fcopy_msg);
> > +
> > +	fcopy_transaction.state = HVUTIL_READY;
> 
> Is the ordering correct here?  

This is intentional. I'll add a comment (please see the below).

> It seems like the fcopy daemon could receive
> the cancel message and do the write before the state is forced to
> HVUTIL_READY.

This can not happen, because when we're here from util_suspend(), all the
userspace processes have been frozen (please refer to another mail from me
https://lkml.org/lkml/2020/1/13/1021). The userspace is thawed only after
util_resume() and the other part of the resume procedure finish.

When we're here in util_suspend(), we can be in any of the below states:

#1: hv_utils has not queued any message to the userspace daemon.
Now hv_fcopy_pre_suspend() queues a message to the daemon, and forces
the state to HVUTIL_READY; the daemon should read the message without 
any error; later when the daemon calls write(), the write() returns -1 because 
fcopy_transaction.state != HVUTIL_USERSPACE_REQ and fcopy_on_msg() 
returns -EINVAL; the daemon responds to the write() error by closing and 
re-opening the dev file, which triggers a reset in the hv_utils driver: see
hvt_op_release() -> hvt_reset() -> fcopy_on_reset(), and later the daemon
registers itself to the hv_utils driver, and everything comes back to normal.

#2: hv_utils has queued a message to the userspace daemon.
Now hv_fcopy_pre_suspend() fails to queue an extra message to the 
daemon, but still forces the state to HVUTIL_READY.

#2.1 the userspace has not read the message.
The daemon reads the queued message and later the write() fails, so the
daemon closes and re-opens the dev file.

#2.2 the userspace has read the message, but has not called write() yet.
The write() fails, so the daemon closes and re-opens the dev file.

#2.3 the userspace has read the message, and has called write().
This is actualy the same as #1.

So, IMO the patch should be handling the scenarios correctly.
 
> > +
> > +	/* tasklet_enable() will be called in hv_fcopy_pre_resume(). */
> > +
> > +	return 0;
> > +err:
> > +	tasklet_enable(&channel->callback_event);
> 
> A nit, but if you did the memory allocation first, you could return -ENOMEM
> immediately on error and avoid the err: label and re-enabling the tasklet.

Good idea! I'll fix this.

> > +	return -ENOMEM;
> > +}
> > ...
> > --- a/drivers/hv/hv_snapshot.c
> > +++ b/drivers/hv/hv_snapshot.c
> > @@ -229,6 +229,7 @@ static void vss_handle_request(struct work_struct
> *dummy)
> >  		vss_transaction.state = HVUTIL_HOSTMSG_RECEIVED;
> >  		vss_send_op();
> >  		return;
> > +
> 
> Gratuitous blank line added?

Will remove it. I probably tried to make the "return;" more noticeable. 
 
> >  	case VSS_OP_GET_DM_INFO:
> >  		vss_transaction.msg->dm_info.flags = 0;
> >  		break;
> > ...
> > +int hv_vss_pre_suspend(void)
> > +{
> > ...
> > +	/* We don't care about the return value. */
> > +	hvutil_transport_send(hvt, vss_msg, sizeof(*vss_msg), NULL);
> > +
> > +	kfree(vss_msg);
> > +
> > +	vss_transaction.state = HVUTIL_READY;
> 
> Same comment about the ordering.

I'll add a comment for this. I may add a long comment in util_suspend()
and add a short comment here.

> > +
> > +	/* tasklet_enable() will be called in hv_vss_pre_resume(). */
> > +
> > +	return 0;
> > +err:
> > +	tasklet_enable(&channel->callback_event);
> > +	return -ENOMEM;
> 
> Same comment about simplifying the error handling applies here.

Will fix this.
 
> > +static int util_suspend(struct hv_device *dev)
> > +{
> > +	struct hv_util_service *srv = hv_get_drvdata(dev);
> > +	int ret = 0;
> > +
> > +	if (srv->util_pre_suspend) {
> > +		ret = srv->util_pre_suspend();
> > +
> 
> Unneeded blank line?

Will remove this.

> > +		if (ret)
> > +			return ret;
> > +	}
> > +
> > +	vmbus_close(dev->channel);
> > +
> > +	return 0;
> > +}
> > +
> > +static int util_resume(struct hv_device *dev)
> > +{
> > +	struct hv_util_service *srv = hv_get_drvdata(dev);
> > +	int ret = 0;
> > +
> > +	if (srv->util_pre_resume) {
> > +		ret = srv->util_pre_resume();
> > +
> 
> Unneeded blank line?

Will remove this.

Thanks,
-- Dexuan