Re: [PATCH] fixup! mm/sparse-vmemmap: introduce CONFIG_VMEMMAP_OPTIMIZATION

[Muchun Song]

> On Sep 16, 2026, at 16:36, Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
> 
> The section-based vmemmap optimization infrastructure is guarded by
> CONFIG_HUGETLB_PAGE_OPTIMIZE_VMEMMAP, but it can also be used by
> ZONE_DEVICE users that set dev_pagemap::vmemmap_shift. Introduce
> CONFIG_VMEMMAP_OPTIMIZATION as a common config for the shared
> infrastructure.
> 
> Select the new option from HUGETLB_PAGE_OPTIMIZE_VMEMMAP and from
> ZONE_DEVICE when the architecture opts in to DAX vmemmap optimization,
> and use it to guard the generic sparse-vmemmap state and helpers.
> 
> Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>
> Acked-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> ---
> Note: When folding this fix into
> "mm/sparse-vmemmap: introduce CONFIG_VMEMMAP_OPTIMIZATION",
> please use this commit message for the resulting patch.

Hi Andrew,

Sashiko found a configuration issue in patch 1 [1].

MSHV_VTL sets a non-zero dev_pagemap::vmemmap_shift for VTL0
memory. However, VMEMMAP_OPTIMIZATION is currently selected only by
DEV_DAX or HUGETLB_PAGE_OPTIMIZE_VMEMMAP.

With ZONE_DEVICE=y and MSHV_VTL=y, but DEV_DAX=n and
HUGETLB_PAGE=n, VMEMMAP_OPTIMIZATION is therefore disabled.
__vmemmap_can_optimize() always returns false in this configuration,
so VTL0 memory uses an unoptimized vmemmap despite its non-zero
vmemmap_shift. On common x86-64 configurations, this can consume up
to about 1.5% of the mapped VTL0 memory for vmemmap metadata.

The report is correct. The fixup in the parent message moves the
conditional selection of VMEMMAP_OPTIMIZATION from DEV_DAX to
ZONE_DEVICE. This covers every ZONE_DEVICE user that sets
dev_pagemap::vmemmap_shift while retaining the architecture opt-in
through ARCH_WANT_OPTIMIZE_DAX_VMEMMAP.

It also removes the DEV_DAX dependency on ZONE_DEVICE that patch 1
added solely to satisfy the Kconfig dependency of
VMEMMAP_OPTIMIZATION.

Andrew, could you please fold this fixup into patch 1,

"mm/sparse-vmemmap: introduce CONFIG_VMEMMAP_OPTIMIZATION"?

[1] https://sashiko.dev/#/patchset/20260916064341.1825793-1-songmuchun%40bytedance.com

Thanks,
Muchun