linux-kernel - Re: [PATCH v2] mm, hugetlb: implement movable_gigantic

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Message-ID: <c309a251-5d14-49a9-a3d7-5c60d100c4eb@kernel.org>
Date: Tue, 25 Nov 2025 11:24:24 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Gregory Price <gourry@...rry.net>, linux-mm@...ck.org,
 akpm@...ux-foundation.org
Cc: kernel-team@...a.com, vbabka@...e.cz, surenb@...gle.com, mhocko@...e.com,
 jackmanb@...gle.com, hannes@...xchg.org, ziy@...dia.com,
 linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
 Alexandru Moise <00moses.alexander00@...il.com>,
 David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v2] mm, hugetlb: implement movable_gigantic_pages sysctl

On 11/21/25 20:27, Gregory Price wrote:
> This reintroduces a concept removed by
> commit d6cb41cc44c6 ("mm, hugetlb: remove hugepages_treat_as_movable sysctl")
> 
> This sysctl provides some flexibility between multiple ZONE_MOVABLE
> use cases
> 
> 1) onlining memory in ZONE_MOVABLE to maintain hotplug compatibility
> 2) onlining memory in ZONE_MOVABLE to make hugepage allocate reliable
> 
> When ZONE_MOVABLE is used to make huge page allocation more reliable,
> disallowing gigantic pages memory in this region is pointless.  If
> hotplug is not a requirement, we can loosen the restrictions to allow
> 1GB gigantic pages in ZONE_MOVABLE.
> 
> Since 1GB can be difficult to migrate / has impacts on compaction /
> defragmentation, we don't enable this by default. Notably, 1GB pages
> can only be migrated if another 1GB page is available - so hot-unplug
> will fail if such a page cannot be found.
> 
> However, since there are scenarios where gigantic pages are migratable,
> we should allow use of these on movable regions.
> 
> Note: Boot-time CMA is not possible for driver-managed hotplug memory,
> as CMA requires the memory to be registered as SystemRAM at boot time.
> Additionally, 1GB huge pages are not supported by THP.
> 
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Alexandru Moise <00moses.alexander00@...il.com>
> Suggested-by: David Rientjes <rientjes@...gle.com>
> Signed-off-by: Gregory Price <gourry@...rry.net>
> Link: https://lore.kernel.org/all/20180201193132.Hk7vI_xaU%25akpm@linux-foundation.org/
> ---
>   Documentation/admin-guide/sysctl/vm.rst | 17 +++++++++++++++++
>   include/linux/hugetlb.h                 |  3 ++-
>   mm/hugetlb.c                            | 11 +++++++++++
>   3 files changed, 30 insertions(+), 1 deletion(-)
> 
> ---
> v2: changelog updates
> 
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index b325bfbc2611..fe3982604b1f 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -54,6 +54,7 @@ Currently, these files are in /proc/sys/vm:
>   - mmap_min_addr
>   - mmap_rnd_bits
>   - mmap_rnd_compat_bits
> +- movable_gigantic_pages
>   - nr_hugepages
>   - nr_hugepages_mempolicy
>   - nr_overcommit_hugepages
> @@ -623,6 +624,22 @@ This value can be changed after boot using the
>   /proc/sys/vm/mmap_rnd_compat_bits tunable
>   
>   
> +movable_gigantic_pages
> +======================
> +
> +This parameter controls whether gigantic pages may be allocated from
> +ZONE_MOVABLE. If set to non-zero, gigantic hugepages can be allocated

"gigantic pages" or "gigantic hugetlb pages"

> +from ZONE_MOVABLE. ZONE_MOVABLE memory may be created via the kernel
> +boot parameter `kernelcore` or via memory hotplug as discussed in
> +Documentation/admin-guide/mm/memory-hotplug.rst.
> +
> +Support may depend on specific architecture.
> +
> +Note that using ZONE_MOVABLE gigantic pages may make features like
> +memory hotremove more unreliable, as migrating gigantic pages is more
> +difficult due to needing larger amounts of phyiscally contiguous memory.

s/phyiscally/physically/ ?

Can we also reference this parameter here from 
Documentation/admin-guide/mm/memory-hotplug.rst?

In particular, there we also talk about gigantic pages and the 
interaction with ZONE_MOVABLE:

"
- Gigantic pages are unmovable, resulting in user space consuming a
   lot of unmovable memory.

- Huge pages are unmovable when an architectures does not support huge
   page migration, resulting in a similar issue as with gigantic pages.
"

And then later, we talk about "Even with ZONE_MOVABLE, there are some 
corner cases where offlining a memory
block might fail:" where this should be spelled out as well.

-- 
Cheers

David