[PATCH V3] panic: Move panic_print before kmsg dumpers

[Michael Kelley (LINUX)]

From: Baoquan He <bhe at redhat.com> Sent: Saturday, January 29, 2022 12:00 AM
> 
> On 01/26/22 at 12:51pm, Petr Mladek wrote:
> > On Mon 2022-01-24 16:57:17, Michael Kelley (LINUX) wrote:
> > > From: Baoquan He <bhe at redhat.com> Sent: Friday, January 21, 2022 8:34 PM
> > > >
> > > > On 01/21/22 at 03:00pm, Michael Kelley (LINUX) wrote:
> > > > > From: Baoquan He <bhe at redhat.com> Sent: Thursday, January 20, 2022 6:31 PM
> > > > > >
> > > > > > On 01/20/22 at 06:36pm, Guilherme G. Piccoli wrote:
> > > > > > > Hi Baoquan, some comments inline below:
> > > > > > >
> > > > > > > On 20/01/2022 05:51, Baoquan He wrote:
> > >
> > > [snip]
> > >
> > > > > > > Do you think it should be necessary?
> > > > > > > How about if we allow users to just "panic_print" with or without the
> > > > > > > "crash_kexec_post_notifiers", then we pursue Petr suggestion of
> > > > > > > refactoring the panic notifiers? So, after this future refactor, we
> > > > > > > might have a much clear code.
> > > > > >
> > > > > > I haven't read Petr's reply in another panic notifier filter thread. For
> > > > > > panic notifier, it's only enforced to use on HyperV platform, excepto of
> > > > > > that, users need to explicitly add "crash_kexec_post_notifiers=1" to enable
> > > > > > it. And we got bug report on the HyperV issue. In our internal discussion,
> > > > > > we strongly suggest HyperV dev to change the default enablement, instead
> > > > > > leave it to user to decide.
> > > > > >
> > > > >
> > > > > Regarding Hyper-V:   Invoking the Hyper-V notifier prior to running the
> > > > > kdump kernel is necessary for correctness.  During initial boot of the
> > > > > main kernel, the Hyper-V and VMbus code in Linux sets up several guest
> > > > > physical memory pages that are shared with Hyper-V, and that Hyper-V
> > > > > may write to.   A VMbus connection is also established. Before kexec'ing
> > > > > into the kdump kernel, the sharing of these pages must be rescinded
> > > > > and the VMbus connection must be terminated.   If this isn't done, the
> > > > > kdump kernel will see strange memory overwrites if these shared guest
> > > > > physical memory pages get used for something else.
> > >
> > > In the Azure cloud, collecting data before crash dumps is a motivation
> > > as well for setting crash_kexec_post_notifiers to true.   That way as
> > > cloud operator we can see broad failure trends, and in specific cases
> > > customers often expect the cloud operator to be able to provide info
> > > about a problem even if they have taken a kdump.  Where did you
> > > envision adding a comment in the code to help clarify these intentions?
> > >
> > > I looked at the code again, and should revise my previous comments
> > > somewhat.   The Hyper-V resets that I described indeed must be done
> > > prior to kexec'ing the kdump kernel.   Most such resets are actually
> > > done via __crash_kexec() -> machine_crash_shutdown(), not via the
> > > panic notifier. However, the Hyper-V panic notifier must terminate the
> > > VMbus connection, because that must be done even if kdump is not
> > > being invoked.  See commit 74347a99e73.
> > >
> > > Most of the hangs seen in getting into the kdump kernel on Hyper-V/Azure
> > > were probably due to the machine_crash_shutdown() path, and not due
> > > to running the panic notifiers prior to kexec'ing the kdump kernel.  The
> > > exception is terminating the VMbus connection, which had problems that
> > > are hopefully now fixed because of adding a timeout.
> >
> > My undestanding is that we could split the actions into three groups:
> >
> >   1. Actions that has to be before kexec'ing kdump kernel, like
> >      resetting physicall memory shared with Hyper-V.
> >
> >      These operation(s) are needed only for kexec and can be done
> >      in kexec.
> >
> >
> >    2. Notify Hyper-V so that, for example, Azure cloud, could collect
> >       data before crash dump.
> >
> >       It is nice to have.
> >
> >       It should be configurable if it is not completely safe. I mean
> >       that there should be a way to disable it when it might increase
> >       the risk that kexec'ing kdump kernel might fail.
> >
> >
> >    3. Some actions are needed only when panic() ends up in the
> >       infinite loop.
> >
> >       For example, unloading vmbus channel. At least the commit
> >       74347a99e73ae00b8385f ("x86/Hyper-V: Unload vmbus channel in
> >       hv panic callback") says that it is done in kdump path
> >       out of box.
> >
> > All these operations are needed and used only when the kernel is
> > running under Hyper-V.
> >
> > My mine intention is to understand if we need 2 or 3 notifier lists
> > or the current one is enough.
> >
> > The 3 notifier lists would be:
> >
> >    + always do (even before kdump)
> >    + optionally do before or after kdump
> >    + needed only when kdump is not called
> 
> Totally agree with above suggesitons for Hyper-V. Cleanup as ablove
> seems necesary. Stuffing them into panic_notifiers package is not
> appropriate.

Baoquan -- if the concept of panic notifiers is broadened as Petr is
proposing, with three different notifier lists, are you OK with the
Hyper-V requirements being met that way?  Having a generic
mechanism seems better to me than adding #ifdef CONFIG_HYPERV
code into panic().

Michael