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Date:   Wed, 9 Sep 2020 14:36:37 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Laurent Dufour <ldufour@linux.ibm.com>,
        Michal Hocko <mhocko@suse.com>, akpm@linux-foundation.org,
        Oscar Salvador <osalvador@suse.de>, rafael@kernel.org,
        nathanl@linux.ibm.com, cheloha@linux.ibm.com,
        stable@vger.kernel.org, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations
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On Wed, Sep 09, 2020 at 02:32:57PM +0200, David Hildenbrand wrote:
> On 09.09.20 14:30, Greg Kroah-Hartman wrote:
> > On Wed, Sep 09, 2020 at 11:24:24AM +0200, David Hildenbrand wrote:
> >>>> I am not sure an enum is going to make the existing situation less
> >>>> messy. Sure we somehow have to distinguish boot init and runtime hotplug
> >>>> because they have different constrains. I am arguing that a) we should
> >>>> have a consistent way to check for those and b) we shouldn't blow up
> >>>> easily just because sysfs infrastructure has failed to initialize.
> >>>
> >>> For the point a, using the enum allows to know in register_mem_sect_under_node() 
> >>> if the link operation is due to a hotplug operation or done at boot time.
> >>>
> >>> For the point b, one option would be ignore the link error in the case the link 
> >>> is already existing, but that BUG_ON() had the benefit to highlight the root issue.
> >>>
> >>
> >> WARN_ON_ONCE() would be preferred  - not crash the system but still
> >> highlight the issue.
> > 
> > Many many systems now run with 'panic on warn' enabled, so that wouldn't
> > change much :(
> > 
> > If you can warn, you can properly just print an error message and
> > recover from the problem.
> 
> Maybe VM_WARN_ON_ONCE() then to detect this during testing?

If you all use that, sure.

> (we basically turned WARN_ON_ONCE() useless with 'panic on warn' getting
> used in production - behaves like BUG_ON and BUG_ON is frowned upon)

Yes we have, but in the end, it's good, those things should be fixed and
not accessable by anything a user can trigger.

thanks,

greg k-h