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Date:   Tue, 16 Jun 2020 22:30:03 +0000
From:   Anchal Agarwal <anchalag@amazon.com>
To:     Roger Pau =?iso-8859-1?Q?Monn=E9?= <roger.pau@citrix.com>
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Subject: Re: [PATCH 06/12] xen-blkfront: add callbacks for PM suspend and
 hibernation]
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On Tue, Jun 16, 2020 at 09:49:25PM +0000, Anchal Agarwal wrote:
> On Thu, Jun 04, 2020 at 09:05:48AM +0200, Roger Pau Monn? wrote:
> > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> > 
> > 
> > 
> > Hello,
> > 
> > On Wed, Jun 03, 2020 at 11:33:52PM +0000, Agarwal, Anchal wrote:
> > >  CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> > >
> > >
> > >
> > >     On Tue, May 19, 2020 at 11:27:50PM +0000, Anchal Agarwal wrote:
> > >     > From: Munehisa Kamata <kamatam@amazon.com>
> > >     >
> > >     > S4 power transition states are much different than xen
> > >     > suspend/resume. Former is visible to the guest and frontend drivers should
> > >     > be aware of the state transitions and should be able to take appropriate
> > >     > actions when needed. In transition to S4 we need to make sure that at least
> > >     > all the in-flight blkif requests get completed, since they probably contain
> > >     > bits of the guest's memory image and that's not going to get saved any
> > >     > other way. Hence, re-issuing of in-flight requests as in case of xen resume
> > >     > will not work here. This is in contrast to xen-suspend where we need to
> > >     > freeze with as little processing as possible to avoid dirtying RAM late in
> > >     > the migration cycle and we know that in-flight data can wait.
> > >     >
> > >     > Add freeze, thaw and restore callbacks for PM suspend and hibernation
> > >     > support. All frontend drivers that needs to use PM_HIBERNATION/PM_SUSPEND
> > >     > events, need to implement these xenbus_driver callbacks. The freeze handler
> > >     > stops block-layer queue and disconnect the frontend from the backend while
> > >     > freeing ring_info and associated resources. Before disconnecting from the
> > >     > backend, we need to prevent any new IO from being queued and wait for existing
> > >     > IO to complete. Freeze/unfreeze of the queues will guarantee that there are no
> > >     > requests in use on the shared ring. However, for sanity we should check
> > >     > state of the ring before disconnecting to make sure that there are no
> > >     > outstanding requests to be processed on the ring. The restore handler
> > >     > re-allocates ring_info, unquiesces and unfreezes the queue and re-connect to
> > >     > the backend, so that rest of the kernel can continue to use the block device
> > >     > transparently.
> > >     >
> > >     > Note:For older backends,if a backend doesn't have commit'12ea729645ace'
> > >     > xen/blkback: unmap all persistent grants when frontend gets disconnected,
> > >     > the frontend may see massive amount of grant table warning when freeing
> > >     > resources.
> > >     > [   36.852659] deferring g.e. 0xf9 (pfn 0xffffffffffffffff)
> > >     > [   36.855089] xen:grant_table: WARNING:e.g. 0x112 still in use!
> > >     >
> > >     > In this case, persistent grants would need to be disabled.
> > >     >
> > >     > [Anchal Changelog: Removed timeout/request during blkfront freeze.
> > >     > Reworked the whole patch to work with blk-mq and incorporate upstream's
> > >     > comments]
> > >
> > >     Please tag versions using vX and it would be helpful if you could list
> > >     the specific changes that you performed between versions. There where
> > >     3 RFC versions IIRC, and there's no log of the changes between them.
> > >
> > > I will elaborate on "upstream's comments" in my changelog in my next round of patches.
> > 
> > Sorry for being picky, but can you please make sure your email client
> > properly quotes previous emails on reply. Note the lack of '>' added
> > to the quoted parts of your reply.
> That was just my outlook probably. Note taken.
> > 
> > >     > +                     }
> > >     > +
> > >     >                       break;
> > >     > +             }
> > >     > +
> > >     > +             /*
> > >     > +              * We may somehow receive backend's Closed again while thawing
> > >     > +              * or restoring and it causes thawing or restoring to fail.
> > >     > +              * Ignore such unexpected state regardless of the backend state.
> > >     > +              */
> > >     > +             if (info->connected == BLKIF_STATE_FROZEN) {
> > >
> > >     I think you can join this with the previous dev->state == XenbusStateClosed?
> > >
> > >     Also, won't the device be in the Closed state already if it's in state
> > >     frozen?
> > > Yes but I think this mostly due to a hypothetical case if during thawing backend switches to Closed state.
> > > I am not entirely sure if that could happen. Could use some expertise here.
> > 
> > I think the frontend seeing the backend in the closed state during
> > restore would be a bug that should prevent the frontend from
> > resuming.
> > 
> > >     > +     /* Kick the backend to disconnect */
> > >     > +     xenbus_switch_state(dev, XenbusStateClosing);
> > >     > +
> > >     > +     /*
> > >     > +      * We don't want to move forward before the frontend is diconnected
> > >     > +      * from the backend cleanly.
> > >     > +      */
> > >     > +     timeout = wait_for_completion_timeout(&info->wait_backend_disconnected,
> > >     > +                                           timeout);
> > >     > +     if (!timeout) {
> > >     > +             err = -EBUSY;
> > >
> > >     Note err is only used here, and I think could just be dropped.
> > >
> > > This err is what's being returned from the function. Am I missing anything?
> > 
> > Just 'return -EBUSY;' directly, and remove the top level variable. You
> > can also use -EBUSY directly in the xenbus_dev_error call. Anyway, not
> > that important.
> > 
> > >     > +             xenbus_dev_error(dev, err, "Freezing timed out;"
> > >     > +                              "the device may become inconsistent state");
> > >
> > >     Leaving the device in this state is quite bad, as it's in a closed
> > >     state and with the queues frozen. You should make an attempt to
> > >     restore things to a working state.
> > >
> > > You mean if backend closed after timeout? Is there a way to know that? I understand it's not good to
> > > leave it in this state however, I am still trying to find if there is a good way to know if backend is still connected after timeout.
> > > Hence the message " the device may become inconsistent state".  I didn't see a timeout not even once on my end so that's why
> > > I may be looking for an alternate perspective here. may be need to thaw everything back intentionally is one thing I could think of.
> > 
> > You can manually force this state, and then check that it will behave
> > correctly. I would expect that on a failure to disconnect from the
> > backend you should switch the frontend to the 'Init' state in order to
> > try to reconnect to the backend when possible.
> > 
> From what I understand forcing manually is, failing the freeze without
> disconnect and try to revive the connection by unfreezing the
> queues->reconnecting to backend [which never got diconnected]. May be even
> tearing down things manually because I am not sure what state will frontend
> see if backend fails to to disconnect at any point in time. I assumed connected.
> Then again if its "CONNECTED" I may not need to tear down everything and start
> from Initialising state because that may not work.
> 
> So I am not so sure about backend's state so much, lets say if  xen_blkif_disconnect fail,
> I don't see it getting handled in the backend then what will be backend's state?
> Will it still switch xenbus state to 'Closed'? If not what will frontend see, 
> if it tries to read backend's state through xenbus_read_driver_state ?
> 
> So the flow be like:
> Front end marks XenbusStateClosing
> Backend marks its state as XenbusStateClosing
>     Frontend marks XenbusStateClosed
>     Backend disconnects calls xen_blkif_disconnect
>        Backend fails to disconnect, the above function returns EBUSY
>        What will be state of backend here? 
>        Frontend did not tear down the rings if backend does not switches the
>        state to 'Closed' in case of failure.
> 
> If backend stays in CONNECTED state, then even if we mark it Initialised in frontend, backend
> won't be calling connect(). {From reading code in frontend_changed}
> IMU, Initialising will fail since backend dev->state != XenbusStateClosed plus
> we did not tear down anything so calling talk_to_blkback may not be needed
> 
> Does that sound correct?
Send that too quickly, I also meant to add XenBusIntialised state should be ok
only if we expect backend will stay in "Connected" state. Also, I experimented
with that notion. I am little worried about the correctness here. 
Can the backend  come to an Unknown state somehow?
> > >     > +     }
> > >     > +
> > >     > +     return err;
> > >     > +}
> > >     > +
> > >     > +static int blkfront_restore(struct xenbus_device *dev)
> > >     > +{
> > >     > +     struct blkfront_info *info = dev_get_drvdata(&dev->dev);
> > >     > +     int err = 0;
> > >     > +
> > >     > +     err = talk_to_blkback(dev, info);
> > >     > +     blk_mq_unquiesce_queue(info->rq);
> > >     > +     blk_mq_unfreeze_queue(info->rq);
> > >     > +     if (!err)
> > >     > +         blk_mq_update_nr_hw_queues(&info->tag_set, info->nr_rings);
> > >
> > >     Bad indentation. Also shouldn't you first update the queues and then
> > >     unfreeze them?
> > > Please correct me if I am wrong, blk_mq_update_nr_hw_queues freezes the queue
> > > So I don't think the order could be reversed.
> > 
> > Regardless of what blk_mq_update_nr_hw_queues does, I don't think it's
> > correct to unfreeze the queues without having updated them. Also the
> > freezing/unfreezing uses a refcount, so I think it's perfectly fine to
> > call blk_mq_update_nr_hw_queues first and then unfreeze the queues.
> > 
> > Also note that talk_to_blkback returning an error should likely
> > prevent any unfreezing, as the queues won't be updated to match the
> > parameters of the backend.
> >
> I think you are right here. Will send out fixes in V2
> > Thanks, Roger.
> > 
> Thanks,
> Anchal
>