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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id bh14sm12070314ejb.104.2021.04.20.02.31.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Apr 2021 02:31:55 -0700 (PDT) From: Vitaly Kuznetsov To: Michael Kelley Cc: 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 In-Reply-To: <1618894089-126662-1-git-send-email-mikelley@microsoft.com> References: <1618894089-126662-1-git-send-email-mikelley@microsoft.com> Date: Tue, 20 Apr 2021 11:31:54 +0200 Message-ID: <87tuo1i9o5.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org Michael Kelley 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 > --- > > 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 -- Vitaly