linux-kernel - Re: [RFC] hugetlb: add memory-hotplug notifier to only allocate for online nodes

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Message-ID: <5c7c607e-079e-4650-be8d-6a1210730b57@kernel.org>
Date: Thu, 6 Nov 2025 11:01:19 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Swaraj Gaikwad <swarajgaikwad1925@...il.com>,
 Muchun Song <muchun.song@...ux.dev>, Oscar Salvador <osalvador@...e.de>,
 David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "open list:HUGETLB SUBSYSTEM" <linux-mm@...ck.org>,
 open list <linux-kernel@...r.kernel.org>
Cc: skhan@...uxfoundation.org, david.hunter.linux@...il.com
Subject: Re: [RFC] hugetlb: add memory-hotplug notifier to only allocate for
 online nodes

On 06.11.25 09:56, Swaraj Gaikwad wrote:
> This patch is a RFC on a proposed change to the hugetlb cgroup subsystem’s
> css allocation function.
> 
> The existing hugetlb_cgroup_css_alloc() uses for_each_node() to allocate
> nodeinfo for all nodes, including those which are not online yet
> (or never will be). This can waste considerable memory on large-node systems.
> The documentation already lists this as a TODO.

We're talking about the

kzalloc_node(sizeof(struct hugetlb_cgroup_per_node), GFP_KERNEL, node_to_alloc);

$ pahole mm/hugetlb_cgroup.o

struct hugetlb_cgroup_per_node {
         long unsigned int          usage[2];             /*     0    16 */

         /* size: 16, cachelines: 1, members: 1 */
         /* last cacheline: 16 bytes */
};

16 bytes on x86_64. So nobody should care here.

Of course, it depends on HUGE_MAX_HSTATE.

IIRC only HUGE_MAX_HSTATE goes crazy on that with effectively 15 entries.

15*8 ~128 bytes.

So with 1024 nodes we would be allocating 128 KiB.


And given that this is for each cgroup (right?) I assume it can add up.

> 
> Proposed Change:
>      Introduce a memory hotplug notifier that listens for MEM_ONLINE
>      events. When a node becomes online, we call the same allocation function
>      but insted of for_each_node(),using for_each_online_node(). This means
>      memory is only allocated for nodes which are online, thus reducing waste.

We have a NODE_ADDING_FIRST_MEMORY now, I'd assume that is more suitable?

> 
> Feedback Requested:
>      - Where in the codebase (which file or section) is it most appropriate to
>        implement and register the memory hotplug notifier for this subsystem?

I'd assume you would have to register in hugetlb_cgroup_css_alloc() and
free in hugetlb_cgroup_css_free().

>      - Are there best practices or patterns for handling the notifier lifecycle,
>        especially for unregistering during cgroup or subsystem teardown?

Not that I can think of some :)

>      - What are the standard methods or tools to test memory hotplug scenarios
>        for cgroups? Are there ways to reliably trigger node online/offline events
>        in a development environment?

You can use QEMU to hotplug memory (pc-dimm device) to a CPU+memory-less node and
to then remove it again. If you disable automatic memory onlining, you should be able to
trigger this multiple times without any issues.

>      - Are there existing test cases or utilities in the kernel tree that would help
>        to verify correct behavior of this change?

Don't think so.

>      - Any suggestions for implementation improvements or cleaner API usage?

I'd assume you'd want to look into NODE_ADDING_FIRST_MEMORY.

-- 
Cheers

David