Re: [PATCH v3 1/5] mm,memory_hotplug: Allocate memmap from the added memory range

[David Hildenbrand]

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