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From:   Dexuan Cui <decui@microsoft.com>
To:     vkuznets <vkuznets@redhat.com>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "sashal@kernel.org" <sashal@kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Michael Kelley <mikelley@microsoft.com>
Subject: RE: [PATCH 1/3] hv_utils: Add the support of hibernation
Thread-Topic: [PATCH 1/3] hv_utils: Add the support of hibernation
Thread-Index: AQHVaYhA3nfAp6qkREmtH6p7KQYwRKcpxtlAgAQ8TICABIF7gIAAUgiAgALprxA=
Date:   Sat, 21 Sep 2019 07:26:34 +0000
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> From: Vitaly Kuznetsov <vkuznets@redhat.com>
> Sent: Thursday, September 19, 2019 3:28 AM
>=20
> Dexuan Cui <decui@microsoft.com> writes:
>=20
> > 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.
>=20
> Makes sense. Or, alternatively, can we postpone hibernation until after
> VSS_OP_THAW?
>=20
> 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.=20

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 avo=
id
an indeterminate dependency.

Thanks,
-- Dexuan