[PATCH V4] notifier/panic: Introduce panic_notifier_filter

[d.hatayama at fujitsu.com]

> On 01/24/22 at 11:48am, Guilherme G. Piccoli wrote:
> > On 24/01/2022 10:59, Baoquan He wrote:
> > > [...]
> > > About pre_dump, if the dump is crash dump, hope those pre_dump notifiers
> > > will be executed under conditional check, e.g only if 'crash_kexec_post_notifiers'
> > > is specified in kernel cmdline.
> >
> > Hi Baoquan, based on Petr's suggestion, I think pre_dump would be
> > responsible for really *non-intrusive/non-risky* tasks and should be
> > always executed in the panic path (before kdump), regardless of
> > "crash_kexec_post_notifiers".
> >
> > The idea is that the majority of the notifiers would be executed in the
> > post_dump portion, and for that, we have the
> > "crash_kexec_post_notifiers" conditional. I also suggest we have
> > blacklist options (based on function names) for both notifiers, in order
> > to make kdump issues debug easier.
> >
> > Do you agree with that? Feel free to comment with suggestions!
> > Cheers,
>
> I would say "please NO" cautiously.
>
> As Petr said, kdump mostly works only if people configure it correctly.
> That's because we try best to switch to kdump kernel from the fragile
> panicked kernel immediately. When we try to add anthing before the switching,
> please consider carefully and ask if that adding is mandatory, otherwise
> switching into kdump kernel may fail. If the answer is yes, the adding
> is needed and welcomed. Othewise, any unnecessary action, including any
> "non-intrusive/non-risky" tasks, would be unwelcomed.
>
> Surely, we don't oppose the "non-intrusive/non-risky" or completely
> "intrusive/risky" action adding before kdump kernel switching, with a
> conditional limitation. When we handle customers' kdump support, we
> explicitly declare we only support normal and default kdump operation.
> If any action which is done before switching into kdump kernel is specified,
> e.g panic_notifier, panic_print, they need take care of their own kdump
> failure.

Sorry for bringing back the past discussion, but how about
reconsidering the following design?

- kdump-specific notifier list within crash_kexec()

I don't think that the current implementation of
crash_kexec_post_notifiers is ideal, where panic() and panic notifier
are used, which contains the code that is unnecessary for kdump, and
it unnecessarily decreases kdump's reliability. The presence of kdump
code in panic() also conversely makes panic()'s code unnecessarily
complicated.

I use crash_kexec_post_notifiers with the understanding that it
affects kdump's reliability to some degree, but at the same time I
want it to be as reliable as possible.

I think introducing kdump-specific notifier list will resolve the
issues caused by dependencies to panic()'s code.

In addition, as I already said in another mail, I want to avoid
invoking any other handlers passed by users other than me, in order to
keep kdump's reliability. For such purpose, I think some feature like
Guilherme's panic notifier filter is needed.