linux-kernel - Re: [PATCH 1/4] mm/memory_hotplug: Update comment for hotplug memory callback priorities

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Message-ID: <429302cf-1574-4263-b1cb-cb4062509a14@redhat.com>
Date: Tue, 19 Aug 2025 21:08:39 +0200
From: David Hildenbrand <david@...hat.com>
To: Dave Jiang <dave.jiang@...el.com>,
 Marc Herbert <marc.herbert@...ux.intel.com>, linux-cxl@...r.kernel.org,
 linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: gregkh@...uxfoundation.org, rafael@...nel.org, dakr@...nel.org,
 dave@...olabs.net, jonathan.cameron@...wei.com, alison.schofield@...el.com,
 vishal.l.verma@...el.com, ira.weiny@...el.com, dan.j.williams@...el.com,
 akpm@...ux-foundation.org
Subject: Re: [PATCH 1/4] mm/memory_hotplug: Update comment for hotplug memory
 callback priorities

On 19.08.25 17:39, Dave Jiang wrote:
> 
> 
> On 8/19/25 2:18 AM, David Hildenbrand wrote:
>> On 19.08.25 05:14, Marc Herbert wrote:
>>>
>>>
>>> On 2025-08-18 07:08, Dave Jiang wrote:
>>>>
>>>>
>>>> On 8/16/25 12:29 AM, David Hildenbrand wrote:
>>>>> On 14.08.25 19:16, Dave Jiang wrote:
>>>>>> Add clarification to comment for memory hotplug callback ordering as the
>>>>>> current comment does not provide clear language on which callback happens
>>>>>> first.
>>>>>>
>>>>>> Signed-off-by: Dave Jiang <dave.jiang@...el.com>
>>>>>> ---
>>>>>>     include/linux/memory.h | 2 +-
>>>>>>     1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/include/linux/memory.h b/include/linux/memory.h
>>>>>> index 40eb70ccb09d..02314723e5bd 100644
>>>>>> --- a/include/linux/memory.h
>>>>>> +++ b/include/linux/memory.h
>>>>>> @@ -116,7 +116,7 @@ struct mem_section;
>>>>>>       /*
>>>>>>      * Priorities for the hotplug memory callback routines (stored in decreasing
>>>>>> - * order in the callback chain)
>>>>>> + * order in the callback chain). The callback ordering happens from high to low.
>>>>>>      */
>>>>>>     #define DEFAULT_CALLBACK_PRI    0
>>>>>>     #define SLAB_CALLBACK_PRI    1
>>>>>
>>>>> "stored in decreasing order in the callback chain"
>>>>>
>>>>> is pretty clear? It's a chain after all that gets called.
>>>>
>>>> I can drop the patch. For some reason when I read it I'm thinking the opposite, and when Marc was also confused I started questioning things.
>>>>
>>>
>>> I think we both found the current comment confusing (even together!)
>>> because:
>>>
>>> - It very briefly alludes to an implementation detail (the chain)
>>>     without really getting into detail. A "chain" could be bi-directional;
>>>     why not? This one is... "most likely" not. Doubt.
>>>
>>
>> Please note that the memory notifier is really just using the generic *notifier chain* mechanism as documented in include/linux/notifier.h.
>>
>> Here is a good summary of that mechanism. I don't quite agree with the "implementation detail" part, but that information might indeed not be required here.
>>
>> https://0xax.gitbooks.io/linux-insides/content/Concepts/linux-cpu-4.html
>>
>>> - Higher priorities can have lower numbers, example: "P1 bugs". Not the
>>>     case here, but this "double standards" situation makes _all_
>>>     priorities suspicious and confusing.
>>>
>>
>> Yes, "priorities" are handled differently in different context.
>>
>>> - Constants that come first in the file are called last.
>>
>> Yes, but that part makes perfect sense to me.
>>   > I would go further than Dave and also drop the "chain" implementation
>>> detail because it makes the reader to think too much.  Not needed and
>>> distracting at this particular point in the file.
>>
>>   > /*
>>>    * Priorities for the hotplug memory callback routines.
>>>    * Invoked from high to low.
>>>    */
>>>
>>>     => Hopefully zero cognitive load.
>>
>> Still confusion about how a high priority translates to a number, maybe?
>>
>> /*
>>   * Priorities for the hotplug memory callback routines. Invoked from
>>   * high to low. Higher priorities corresponds to higher numbers.
>>   */
>>
> 
> This reads clear to me. I will adopt this comment if there are no additional objections.


Feel free to add

Acked-by: David Hildenbrand <david@...hat.com>


-- 
Cheers

David / dhildenb