Re: [RFC PATCH 0/6] mm/hugetlb: Dynamic, NUMA-aware HugePage Cache & Free Page Reporting

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 07, 2026 at 06:42:29AM +0000, Sourav Panda wrote:
> Overview
> This patch series introduces a dynamic, NUMA-aware HugePage Cache,
> backed by a kernel shrinker to safely return memory under pressure, and
> integrates it with Free Page Reporting (virtio-balloon) for HugeTLB,
> specifically targeting gigantic (1GB) hugepages. The goal is to solve
> the tradeoff between allocation latency and memory
> fungibility in virtualized and heterogeneous cloud environments.
> 
> ---
> 
> The Core Problem: Allocation Latency vs. Memory Fungibility
> 
> With highly heterogeneous workloads, latency-critical applications demand
> gigantic hugepages. However, dynamic runtime allocation of 1GB pages
> from the buddy allocator (via CMA) is slow.
> 
> To bypass this latency, operators often pre-allocate hugepages
> statically. However, this locks up the memory: when the HugeTLB
> workloads are idle, that memory is completely unavailable for other
> buddy-allocator workloads (e.g., page cache, anonymous memory). If buddy
> memory is exhausted, the system will OOM even if gigabytes of HugeTLB
> pages are sitting idle.
> 
> This series resolves this challenge by delivering Dynamic Fungibility:
> 
> 1.  Dynamic Caching: Intercepts freed surplus hugepages and recycles
>     them into a NUMA-aware cache instead of dissolving them immediately.
> 2.  Fast Allocations: Satisfies subsequent dynamic allocations
>     instantly from this warm, local hugepage cache.
> 3.  Kernel Shrinker Integration: Registers a NUMA-aware kernel shrinker
>     to dynamically dissolve cached pages back to the buddy allocator
>     under memory pressure, restoring host/guest memory fungibility.
> 4.  Free Page Reporting Integration: For virtualized environments (Guest
>     VMs),

I was going to look into this part, thanks for working on this.

> cached pages trigger background Free Page Reporting via
>     virtio-balloon. This allows the host to reclaim the physical memory
>     while the guest retains its Vmemmap Optimization (HVO) metadata
>     savings (~14GB saved per 1TB VM)!

Why "!" - that's 1.5%, seems surprisingly modest.
Are you sure it's working as intended?

>   +---------------+   Slow Allocate  +---------------------+
>   | Buddy         | ---------------> | Active HugeTLB Page |
>   | Allocator     |                  +---------------------+
>   +---------------+                    |                ^
>     ^                                  | (1) Free to    | (2) Fast
>     | (3) Under                        v     Cache      |     Allocate
>     | Pressure                       +---------------------+
>     | Shrink()  <------------------- | HugeTLB Cache       |
>                                      +---------------------+
>                                        |
>                                        v (4) Free Page Reporting
>                                          (Host Reclaim in the case
>                                           of virtualization)
> 
> ---
> 
> Patch Series Structure
> 
> Patch 1/6: mm/hugetlb: add Kconfig and basic cache infrastructure
>   - Introduces the CONFIG_HUGETLB_CACHE option, hstate tracking fields,
>     and the HPG_cached page flag.
>   - Establishes the clean helper API (hugetlb_folio_is_cached(),
>     hugetlb_cache_remove(), hugetlb_cache_add()) and updates
>     remove_hugetlb_folio() to be cache-aware, eliminating inline #ifdef
>     blocks.
> Patch 2/6: mm/hugetlb: implement cache recycling and allocation
>   - Hooks up recycling in free_huge_folio() (up to the cache limit) and
>     allocation in alloc_surplus_hugetlb_folio().
>   - Implements MRU allocation policy for maximum warmth, poison safety
>     checks, and MTE/dcache cleaning.
> Patch 3/6: mm/hugetlb: add sysfs interfaces for cache
>   - Exposes global and per-node sysfs attributes (max_cached_huge_pages,
>     nr_cached_hugepages) for dynamic userspace control, including NUMA
>     memory policy scaling.
>   - Supports dynamic delta adjustments (+1/-1) to safely scale cache
>     sizes alongside concurrent background reclaim operations.
> Patch 4/6: mm/hugetlb: add memory shrinker for cache
>   - Registers a NUMA-aware kernel shrinker to evict and dissolve cached
>     gigantic pages back to buddy under memory pressure.
> Patch 5/6: Documentation/admin-guide/mm/hugetlbpage.rst: document cache
>     interfaces
>   - Documents the Kconfig option, sysfs attributes, and shrinker
>     behavior in the admin guide.
> Patch 6/6: mm/hugetlb: support free page reporting for cached hugepages
>   - Integrates the cache with the Free Page Reporting framework (virtio-
>     balloon), introducing the HPG_reported flag and the
>     reporting/isolation/draining lifecycle.
> Sourav Panda (6):
>   mm/hugetlb: add Kconfig and basic cache infrastructure
>   mm/hugetlb: implement cache recycling and allocation
>   mm/hugetlb: add sysfs interfaces for cache
>   mm/hugetlb: add memory shrinker for cache
>   Documentation/admin-guide/mm/hugetlbpage.rst: document cache
>     interfaces
>   mm/hugetlb: support free page reporting for cached hugepages
> 
>  Documentation/admin-guide/mm/hugetlbpage.rst |  34 +-
>  fs/Kconfig                                   |   9 +
>  include/linux/hugetlb.h                      |  45 ++
>  include/linux/page_reporting.h               |   1 +
>  mm/hugetlb.c                                 | 590 ++++++++++++++++++-
>  mm/hugetlb_internal.h                        |   9 +
>  mm/hugetlb_sysfs.c                           | 158 +++++
>  mm/page_reporting.c                          |  10 +-
>  mm/page_reporting.h                          |   6 +
>  9 files changed, 844 insertions(+), 18 deletions(-)
> 
> -- 
> 2.55.0.rc0.799.gd6f94ed593-goog