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

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Message-ID: <41ec1e23-e0f6-4275-ba9b-a34c2cbddbd9@intel.com>
Date: Tue, 19 Aug 2025 08:39:55 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: David Hildenbrand <david@...hat.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 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.