linux-kernel - Re: ** POTENTIAL FRAUD ALERT - RED HAT ** [PATCH v2 1/1] Drivers: hv: vmbus: Increase wait time for VMbus unload

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Date:   Tue, 20 Apr 2021 11:31:54 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Michael Kelley <mikelley@...rosoft.com>
Cc:     mikelley@...rosoft.com, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, wei.liu@...nel.org,
        linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        decui@...rosoft.com
Subject: Re: ** POTENTIAL FRAUD ALERT - RED HAT ** [PATCH v2 1/1] Drivers:
 hv: vmbus: Increase wait time for VMbus unload

Michael Kelley <mikelley@...rosoft.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@...rosoft.com>
> ---
>
> Changed in v2: Fixed silly error in the argument to mdelay()
>
> ---
>  drivers/hv/channel_mgmt.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index f3cf4af..ef4685c 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -755,6 +755,12 @@ static void init_vp_index(struct vmbus_channel *channel)
>  	free_cpumask_var(available_mask);
>  }
>  
> +#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.

>  	 */
> -	for (i = 0; i < 1000; i++) {
> +	for (i = 1; i <= UNLOAD_WAIT_LOOPS; i++) {
>  		if (completion_done(&vmbus_connection.unload_event))
> -			break;
> +			goto completed;
>  
>  		for_each_online_cpu(cpu) {
>  			struct hv_per_cpu_context *hv_cpu
> @@ -800,9 +811,18 @@ static void vmbus_wait_for_unload(void)
>  			vmbus_signal_eom(msg, message_type);
>  		}
>  
> -		mdelay(10);
> +		/*
> +		 * Give a notice periodically so someone watching the
> +		 * serial output won't think it is completely hung.
> +		 */
> +		if (!(i % UNLOAD_MSG_LOOPS))
> +			pr_notice("Waiting for VMBus UNLOAD to complete\n");
> +
> +		mdelay(UNLOAD_DELAY_UNIT_MS);
>  	}
> +	pr_err("Continuing even though VMBus UNLOAD did not complete\n");
>  
> +completed:
>  	/*
>  	 * We're crashing and already got the UNLOAD_RESPONSE, cleanup all
>  	 * maybe-pending messages on all CPUs to be able to receive new

This is definitely an improvement,

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly