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Date:   Tue, 20 Apr 2021 19:46:42 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     Michael Kelley <mikelley@microsoft.com>, kys@microsoft.com,
        haiyangz@microsoft.com, sthemmin@microsoft.com, wei.liu@kernel.org,
        linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
        decui@microsoft.com
Subject: Re: ** POTENTIAL FRAUD ALERT - RED HAT ** [PATCH v2 1/1] Drivers:
 hv: vmbus: Increase wait time for VMbus unload
Message-ID: <20210420194642.hnonoj4fftmktciy@liuwe-devbox-debian-v2>
References: <1618894089-126662-1-git-send-email-mikelley@microsoft.com>
 <87tuo1i9o5.fsf@vitty.brq.redhat.com>
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On Tue, Apr 20, 2021 at 11:31:54AM +0200, Vitaly Kuznetsov wrote:
> Michael Kelley <mikelley@microsoft.com> writes:
> 
> > When running in Azure, disks may be connected to a Linux VM with
> > read/write caching enabled. If a VM panics and issues a VMbus
> > UNLOAD request to Hyper-V, the response is delayed until all dirty
> > data in the disk cache is flushed.  In extreme cases, this flushing
> > can take 10's of seconds, depending on the disk speed and the amount
> > of dirty data. If kdump is configured for the VM, the current 10 second
> > timeout in vmbus_wait_for_unload() may be exceeded, and the UNLOAD
> > complete message may arrive well after the kdump kernel is already
> > running, causing problems.  Note that no problem occurs if kdump is
> > not enabled because Hyper-V waits for the cache flush before doing
> > a reboot through the BIOS/UEFI code.
> >
> > Fix this problem by increasing the timeout in vmbus_wait_for_unload()
> > to 100 seconds. Also output periodic messages so that if anyone is
> > watching the serial console, they won't think the VM is completely
> > hung.
> >
> > Fixes: 911e1987efc8 ("Drivers: hv: vmbus: Add timeout to vmbus_wait_for_unload")
> > Signed-off-by: Michael Kelley <mikelley@microsoft.com>

Applied to hyperv-next. Thanks.

> > ---
[...]
> >  
> > +#define UNLOAD_DELAY_UNIT_MS	10		/* 10 milliseconds */
> > +#define UNLOAD_WAIT_MS		(100*1000)	/* 100 seconds */
> > +#define UNLOAD_WAIT_LOOPS	(UNLOAD_WAIT_MS/UNLOAD_DELAY_UNIT_MS)
> > +#define UNLOAD_MSG_MS		(5*1000)	/* Every 5 seconds */
> > +#define UNLOAD_MSG_LOOPS	(UNLOAD_MSG_MS/UNLOAD_DELAY_UNIT_MS)
> > +
> >  static void vmbus_wait_for_unload(void)
> >  {
> >  	int cpu;
> > @@ -772,12 +778,17 @@ static void vmbus_wait_for_unload(void)
> >  	 * vmbus_connection.unload_event. If not, the last thing we can do is
> >  	 * read message pages for all CPUs directly.
> >  	 *
> > -	 * Wait no more than 10 seconds so that the panic path can't get
> > -	 * hung forever in case the response message isn't seen.
> > +	 * Wait up to 100 seconds since an Azure host must writeback any dirty
> > +	 * data in its disk cache before the VMbus UNLOAD request will
> > +	 * complete. This flushing has been empirically observed to take up
> > +	 * to 50 seconds in cases with a lot of dirty data, so allow additional
> > +	 * leeway and for inaccuracies in mdelay(). But eventually time out so
> > +	 * that the panic path can't get hung forever in case the response
> > +	 * message isn't seen.
> 
> I vaguely remember debugging cases when CHANNELMSG_UNLOAD_RESPONSE never
> arrives, it was kind of pointless to proceed to kexec as attempts to
> reconnect Vmbus devices were failing (no devices were offered after
> CHANNELMSG_REQUESTOFFERS AFAIR). Would it maybe make sense to just do
> emergency reboot instead of proceeding to kexec when this happens? Just
> wondering.
> 

Please submit a follow-up patch if necessary.

Wei.