RE: [PATCH 1/3] hv_utils: Add the support of hibernation

[Dexuan Cui <decui@microsoft.com>]

> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> Sent: Thursday, September 19, 2019 3:28 AM
> 
> Dexuan Cui <decui@microsoft.com> writes:
> 
> > BTW, for vss, maybe the VM should not hibernate if there is a backup
> > ongoing? -- if the file system is frozen by hv_vss_daemon, and the VM
> > hibernates, then when the VM resumes back, it's almost always true that
> > the VM won't receive the host's VSS_OP_THAW request, and the VM will
> > end up in an unusable state.
> 
> Makes sense. Or, alternatively, can we postpone hibernation until after
> VSS_OP_THAW?
> 
> Vitaly

It looks we should not postpone that, because:
1. When we're in util_suspend(), all the user space processes have been
frozen, so even if the VM receives the VSS_OP_THAW message form the host,
there is no chance for the hv_vss_daemon to handle it. 

2. Between the window the host sends the VSS_OP_FREEZE message and the
VSS_OP_THAW mesasge, util_suspend() may jump in and close the channel,
and then the host will not send a VSS_OP_THAW.

3. The host doesn't guarantee how soon it sends the VSS_OP_THAW message,
though in practice IIRC the host *usually* sends the message soon. The
hibernation process has a watchdog of 120s set by dpm_watchdog_set(): if
dpm_suspend() (which calls util_probe()) can not finish in 120 seconds, the
hibernation will be aborted.

3 may not look like a strong reason, but generally speaking I'd like to avoid
an indeterminate dependency.

Thanks,
-- Dexuan