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 8/3/23 5:00 PM, Michal Hocko wrote:
> On Thu 03-08-23 11:24:08, David Hildenbrand wrote:
> [...]
>>> would be readable only when the block is offline and it would reallocate
>>> vmemmap on the change. Makes sense? Are there any risks? Maybe pfn
>>> walkers?
>>
>> The question is: is it of any real value such that it would be worth the
>> cost and risk?
>>
>>
>> One of the primary reasons for memmap_on_memory is that *memory hotplug*
>> succeeds even in low-memory situations (including, low on ZONE_NORMAL
>> situations).
> 
> One usecase I would have in mind is a mix of smaller and larger memory
> blocks. For larger ones you want to have memmap_on_memory in general
> because they do not eat memory from outside but small(er) ones might be
> more tricky because now you can add a lot of blocks that would be
> internally fragmented to prevent larger allocations to form.
> 


I guess that closely aligns with device memory and being able to add
device memory via dax/kmem using a larger memory block size.
We can then make sure we enable the memmap_on_memory feature
at the device level for this device memory. Do you see a need for
firmware-managed memory to be hotplugged in with different memory block sizes?



>> So you want that behavior already when hotplugging such
>> devices. While there might be value to relocate it later, I'm not sure if
>> that is really worth it, and it does not solve the main use case.
> 
> Is it worth it? TBH I am not sure same as I am not sure the global
> default should be writable after boot. If we want to make it more
> dynamic we should however talk about the proper layer this is
> implemented on.

-aneesh