Re: [PATCH 2/5] mm,memory_hotplug: Allocate memmap from the added memory range

Oscar Salvador
2021-01-26T02:56:46Z

On Mon, Jan 25, 2021 at 11:57:20AM +0100, David Hildenbrand wrote:
> I'm confused.
>
> 1. Assume we hotplug memory, online it to ZONE_MOVABLE. The vmemmap gets
> allocated from altmap space.

The vmemmap could have never been allocated from altmap in case hpage vmemmap
feature is enabled.

Have a look at [1].
If is_hugetlb_free_vmemmap_enabled(), vmemmap_populate() ends up calling
vmemmap_populate_basepages().

And since no memory was consumed from altmap, and hence altmap_alloc_block_buf()
was never called, vmem_altmap->alloc will be 0, and memory_block->nr_vmemmap_pages
will be 0 as well.

But on a second though, true is that we will get in trouble if hpage vmemmap
feature ever gets to work with vmemmap_populate_hugepages.
I will queue that to look in a new future.

[1] https://patchwork.kernel.org/project/linux-mm/patch/20210117151053.24600-10-songmuchun@bytedance.com/

--
Oscar Salvador
SUSE L3