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

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Date:   Mon, 25 Jan 2021 12:18:17 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        akpm@...ux-foundation.org, mhocko@...nel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, vbabka@...e.cz,
        pasha.tatashin@...een.com
Subject: Re: [PATCH 2/5] mm,memory_hotplug: Allocate memmap from the added
 memory range

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