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Subject: Re: [PATCH] mm: don't rely on system state to detect hot-plug
 operations
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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>
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From:   David Hildenbrand <david@redhat.com>
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Message-ID: <e3ea2aab-70d5-0da4-7e72-c02051854497@redhat.com>
Date:   Wed, 9 Sep 2020 14:32:57 +0200
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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?

(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)

-- 
Thanks,

David / dhildenb