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

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Message-ID: <83a930e5-660e-49ed-8c34-66c4edf93665@redhat.com>
Date: Tue, 19 Aug 2025 11:18:29 +0200
From: David Hildenbrand <david@...hat.com>
To: Marc Herbert <marc.herbert@...ux.intel.com>,
 Dave Jiang <dave.jiang@...el.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 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.
  */

-- 
Cheers

David / dhildenb