[PATCH v3 4/7] mm/hotplug: Allow pageblock alignment via altmap reservation

David Hildenbrand david at redhat.com
Wed Jul 12 17:22:56 AEST 2023

On 12.07.23 05:16, Aneesh Kumar K V wrote:
> On 7/11/23 10:49 PM, David Hildenbrand wrote:
>> On 11.07.23 06:48, Aneesh Kumar K.V wrote:
>>> Add a new kconfig option that can be selected if we want to allow
>>> pageblock alignment by reserving pages in the vmemmap altmap area.
>>> This implies we will be reserving some pages for every memoryblock
>>> This also allows the memmap on memory feature to be widely useful
>>> with different memory block size values.
>> "reserving pages" is a nice way of saying "wasting memory". :) Let's spell that out.
>> I think we have to find a better name for this, and I think we should have a toggle similar to memory_hotplug.memmap_on_memory. This should be an admin decision, not some kernel config option.
>> memory_hotplug.force_memmap_on_memory
>> "Enable the memmap on memory feature even if it could result in memory waste due to memmap size limitations. For example, if the memmap for a memory block requires 1 MiB, but the pageblock size is 2 MiB, 1 MiB
>> of hotplugged memory will be wasted. Note that there are still cases where the feature cannot be enforced: for example, if the memmap is smaller than a single page, or if the architecture does not support the forced mode in all configurations."
>> Thoughts?
> With module parameter, do we still need the Kconfig option?


Sleeping over this, maybe we can convert the existing 
memory_hotplug.memmap_on_memory parameter to also accept "force".


David / dhildenb

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