RE: [PATCH] Drivers: hv: Change flag to write log level in panic msg to false

[Joseph Salisbury <Joseph.Salisbury@microsoft.com>]

Thanks for the feedback, Michael.  I'll send a v2.

Thanks,

Joe


-----Original Message-----
From: Michael Kelley <mikelley@microsoft.com> 
Sent: Friday, June 26, 2020 4:53 PM
To: Joseph Salisbury <Joseph.Salisbury@microsoft.com>; KY Srinivasan <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>; sashal@kernel.org; wei.liu@kernel.org
Cc: linux-hyperv@vger.kernel.org; linux-kernel@vger.kernel.org; stable@vger.kernel.org
Subject: RE: [PATCH] Drivers: hv: Change flag to write log level in panic msg to false

From: Joseph Salisbury <joseph.salisbury@microsoft.com> Sent: Friday, June 26, 2020 10:48 AM
> 
> When the kernel panics, one page worth of kmsg data is written to an 
> allocated page.  The Hypervisor is notified of the page address trough 
> the MSR.  This panic information is collected on the host.  Since we 
> are only collecting one page of data, the full panic message may not be collected.
> 
> Each line of the panic message is prefixed with the log level of that
> particular message in the form <N>, where N is the log level.   The typical
> 4 Kbytes contains anywhere from 50 to 100 lines with that log level prefix.
> 
> hv_dmsg_dump() makes a call to kmsg_dump_get_buffer().  The second 
> argument in the call is a bool described as: ‘@syslog: include the “<4>” Prefixes’.
> 
> With this change, we will not write the log level to the allocated 
> page.  This will provide additional room in the allocated page for 
> more informative panic information.

Let me suggest tightening the commit message a bit, with focus on the "what"
and "why" rather than the details of the code change.  Also use imperative voice per the Linux kernel guidelines:

When the kernel panics, one page of kmsg data may be collected and sent to Hyper-V to aid in diagnosing the failure.  The collected kmsg data typically contains 50 to 100 lines, each of which has a log level prefix that isn't very useful from a diagnostic standpoint.  So tell kmsg_dump_get_buffer() to not include the log level, enabling more information that *is* useful to fit in the page.

> 
> Requesting in stable kernels, since many kernels running in production 
> are stable releases.
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Joseph Salisbury <joseph.salisbury@microsoft.com>
> ---
>  drivers/hv/vmbus_drv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c index 
> 9147ee9d5f7d..d69f4efa3719 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1368,7 +1368,7 @@ static void hv_kmsg_dump(struct kmsg_dumper *dumper,
>  	 * Write dump contents to the page. No need to synchronize; panic should
>  	 * be single-threaded.
>  	 */
> -	kmsg_dump_get_buffer(dumper, true, hv_panic_page, HV_HYP_PAGE_SIZE,
> +	kmsg_dump_get_buffer(dumper, false, hv_panic_page, HV_HYP_PAGE_SIZE,
>  			     &bytes_written);
>  	if (bytes_written)
>  		hyperv_report_panic_msg(panic_pa, bytes_written);
> --
> 2.17.1

With the commit message changes,

Reviewed-by: Michael Kelley <mikelley@microsoft.com>