Re: [PATCH] Only allow to set crash_kexec_post_notifiers on boot time

From: Philipp Rudo <prudo@linux.ibm.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	bhe@redhat.com, kexec@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Eric Biederman <ebiederm@xmission.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Eric DeVolder <eric.devolder@oracle.com>,
	Dave Young <dyoung@redhat.com>
Subject: Re: [PATCH] Only allow to set crash_kexec_post_notifiers on boot time
Date: Tue, 22 Sep 2020 12:58:09 +0200	[thread overview]
Message-ID: <20200922125809.66690d54@ibm> (raw)
In-Reply-To: <20200921201811.GB3437@char.us.oracle.com>

Hi Konrad,

On Mon, 21 Sep 2020 16:18:12 -0400
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:

> On Fri, Sep 18, 2020 at 05:47:43PM -0700, Andrew Morton wrote:
> > On Fri, 18 Sep 2020 11:25:46 +0800 Dave Young <dyoung@redhat.com> wrote:
> >   
> > > crash_kexec_post_notifiers enables running various panic notifier
> > > before kdump kernel booting. This increases risks of kdump failure.
> > > It is well documented in kernel-parameters.txt. We do not suggest
> > > people to enable it together with kdump unless he/she is really sure.
> > > This is also not suggested to be enabled by default when users are
> > > not aware in distributions.
> > > 
> > > But unfortunately it is enabled by default in systemd, see below
> > > discussions in a systemd report, we can not convince systemd to change
> > > it:
> > > https://github.com/systemd/systemd/issues/16661
> > > 
> > > Actually we have got reports about kdump kernel hangs in both s390x
> > > and powerpcle cases caused by the systemd change,  also some x86 cases
> > > could also be caused by the same (although that is in Hyper-V code
> > > instead of systemd, that need to be addressed separately).  
> 
> Perhaps it may be better to fix the issus on s390x and PowerPC as well?

There's little s390 can fix. We use the panic_notifier_list to start
other dumpers in case kdump isn't configured or failed. This behavior was
introduced in 2006 long before crash_kexec_post_notifiers were introduced. So I
suggest that crash_kexec_post_notifiers are fixed instead.

> > > 
> > > Thus to avoid the auto enablement here just disable the param writable
> > > permission in sysfs.
> > >   
> > 
> > Well.  I don't think this is at all a desirable way of resolving a
> > disagreement with the systemd developers
> > 
> > At the above github address I'm seeing "ryncsn added a commit to
> > ryncsn/systemd that referenced this issue 9 days ago", "pstore: don't
> > enable crash_kexec_post_notifiers by default".  So didn't that address
> > the issue?  
> 
> It does in systemd, but there is a strong interest in making this on by default.

AFAIK pstore requires UEFI to work. So what's the point to enable it on non-UEFI
systems?

Thanks
Philipp