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SubjectRe: [PATCH v3 1/5] mm,memory_hotplug: Allocate memmap from the added memory range
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On 08.03.21 15:04, Oscar Salvador wrote:
> On Sun, Mar 07, 2021 at 10:16:36PM -0500, Zi Yan wrote:
>> +Mike for hugetlb discussion.
>>
>> Just thinking about how it might impact gigantic page allocation like hugetlb.
>> When MHP_MEMMAP_ON_MEMORY is on, memmap pages are placed at the beginning
>> of each hot added memory block, so available PFNs from two consecutive
>> hot added memory blocks are not all contiguous, separated by memmap pages.
>> If the memory block size is <= 1GB, there is no way of reserving gigantic
>> pages for hugetlb during runtime using alloc_contig_pages from any hot
>> added memory. Am I getting this right?
>
> Yes, that is why it is stated both in boot parameter documentation and
> patch changelog that this feature does not play well in those setups
> where your workload is in need of large contiguous chunks of memory,
> that being gigantic hugetlb or just normal memory.
>
>> I see this implication is documented at the high level in patch 3. Just
>> wonder if we want to be more specific. Or hugetlb is rarely used along
>> with hot-add memory.
>
> I think it is quite normal to see hugetlb and hotplug operations in the
> same environment.
> One thing excludes the other, just need to be careful when it comes to
> potential pitfalls during offline operations.
>
> I guess we could mention hugetlb pages in the documentation, if it feels
> it is necesary.

Runtime allocation of gigantic pages without CMA is absolutely
unreliable either way. IMHO, a tunable for the admin is good enough.

--
Thanks,

David / dhildenb

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