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 8/2/23 9:24 PM, David Hildenbrand wrote:
> On 02.08.23 17:50, Michal Hocko wrote:
>> On Wed 02-08-23 10:15:04, Aneesh Kumar K V wrote:
>>> 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.
>>
>>
>> I do not really see a solid argument why rebooting is really a problem
>> TBH. Also is the global policy knob really the right fit for existing
>> hotplug usecases? In other words, if we really want to make
>> memmap_on_memory more flexible would it make more sense to have it per
>> memory block property instead (the global knob being the default if
>> admin doesn't specify it differently).
> 
> Per memory block isn't possible, due to the creation order. Also, I think it's not the right approach.
> 
> I thought about driver toggles. At least for dax/kmem people are looking into that:
> 
> https://lkml.kernel.org/r/20230801-vv-kmem_memmap-v3-2-406e9aaf5689@intel.com
> 
> Where that can also be toggled per device.
> 

That still is dependent on the global memmap_on_memory enabled right? We could make the dax facility independent of the
global feature toggle? 

-aneesh