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

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Message-ID: <20251106085645.13607-1-swarajgaikwad1925@gmail.com>
Date: Thu,  6 Nov 2025 08:56:41 +0000
From: Swaraj Gaikwad <swarajgaikwad1925@...il.com>
To: Muchun Song <muchun.song@...ux.dev>,
	Oscar Salvador <osalvador@...e.de>,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm@...ck.org (open list:HUGETLB SUBSYSTEM),
	linux-kernel@...r.kernel.org (open list)
Cc: skhan@...uxfoundation.org,
	david.hunter.linux@...il.com,
	Swaraj Gaikwad <swarajgaikwad1925@...il.com>
Subject: [RFC] hugetlb: add memory-hotplug notifier to only allocate for online nodes

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.

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.

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?
    - Are there best practices or patterns for handling the notifier lifecycle,
      especially for unregistering during cgroup or subsystem teardown?
    - 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?
    - Are there existing test cases or utilities in the kernel tree that would help
      to verify correct behavior of this change?
    - Any suggestions for implementation improvements or cleaner API usage?

Thanks,
Swaraj Gaikwad <swarajgaikwad1925@...il.com>

Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@...il.com>