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

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Message-ID: <MW2PR2101MB1052837695100EF9B4BC9874D7930@MW2PR2101MB1052.namprd21.prod.outlook.com>
Date:   Fri, 26 Jun 2020 22:23:38 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Joseph Salisbury <Joseph.Salisbury@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>
CC:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] Drivers: hv: Change flag to write log level in panic msg
 to false

From: Joseph Salisbury <Joseph.Salisbury@...rosoft.com>  Sent: Friday, June 26, 2020 3:16 PM
> 
> Thanks for the feedback, Michael.  I'll send a v2.
> 
> Thanks,
> 
> Joe

A quick note:  The style on the Linux kernel mailing lists is to always reply
inline, after the text you are replying to.  This is quite different from the
style usually used inside Microsoft, which is called "top posting".

Michael

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