Re: [PATCH v7 7/7] mm/memory_hotplug: Enable runtime update of memmap_on_memory parameter

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 8/1/23 4:20 PM, Michal Hocko wrote:
> On Tue 01-08-23 14:58:29, Aneesh Kumar K V wrote:
>> On 8/1/23 2:28 PM, Michal Hocko wrote:
>>> On Tue 01-08-23 10:11:16, Aneesh Kumar K.V wrote:
>>>> Allow updating memmap_on_memory mode after the kernel boot. Memory
>>>> hotplug done after the mode update will use the new mmemap_on_memory
>>>> value.
>>>
>>> Well, this is a user space kABI extension and as such you should spend
>>> more words about the usecase. Why we could live with this static and now
>>> need dynamic?
>>>
>>
>> This enables easy testing of memmap_on_memory feature without a kernel reboot.
> 
> Testing alone is rather weak argument to be honest.
> 
>> I also expect people wanting to use that when they find dax kmem memory online
>> failing because of struct page allocation failures[1]. User could reboot back with
>> memmap_on_memory=y kernel parameter. But being able to enable it via sysfs makes
>> the feature much more useful.
> 
> Sure it can be useful but that holds for any feature, right. The main
> question is whether this is worth maintaing. The current implementation
> seems rather trivial which is an argument to have it but are there any
> risks long term? Have you evaluated a potential long term maintenance
> cost? There is no easy way to go back and disable it later on without
> breaking some userspace.
> 
> All that should be in the changelog!

I updated it as below. 

mm/memory_hotplug: Enable runtime update of memmap_on_memory parameter

Allow updating memmap_on_memory mode after the kernel boot. Memory
hotplug done after the mode update will use the new mmemap_on_memory
value.

It is now possible to test the memmap_on_memory feature easily without
the need for a kernel reboot. Additionally, for those encountering
struct page allocation failures while using dax kmem memory online, this
feature may prove useful. Instead of rebooting with the
memmap_on_memory=y kernel parameter, users can now enable it via sysfs,
which greatly enhances its usefulness. Making the necessary changes to
support runtime updates is a simple change that should not affect the
addition of new features or result in long-term maintenance problems.