Re: [PATCH v1] docs: filesystems: clarify KernelPageSize vs. MMUPageSize in smaps

[Zi Yan]

On 4 Mar 2026, at 10:56, David Hildenbrand (Arm) wrote:

> There was recently some confusion around THPs and the interaction with
> KernelPageSize / MMUPageSize. Historically, these entries always
> correspond to the smallest size we could encounter, not any current
> usage of transparent huge pages or larger sizes used by the MMU.
>
> Ever since we added THP support many, many years ago, these entries
> would keep reporting the smallest (fallback) granularity in a VMA.
>
> For this reason, they default to PAGE_SIZE for all VMAs except for
> VMAs where we have the guarantee that the system and the MMU will
> always use larger page sizes. hugetlb, for example, exposes a custom
> vm_ops->pagesize callback to handle that. Similarly, dax/device
> exposes a custom vm_ops->pagesize callback and provides similar
> guarantees.
>
> Let's clarify the historical meaning of KernelPageSize / MMUPageSize,
> and point at "AnonHugePages", "ShmemPmdMapped" and "FilePmdMapped"
> regarding PMD entries.
>
> While at it, document "FilePmdMapped", clarify what the "AnonHugePages"
> and "ShmemPmdMapped" entries really mean, and make it clear that there
> are no other entries for other THP/folio sizes or mappings.
>
> Link: https://lore.kernel.org/all/20260225232708.87833-1-ak@xxxxxxxxxxxxxxx/
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Cc: Nico Pache <npache@xxxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx
> Cc: Dev Jain <dev.jain@xxxxxxx>
> Cc: Barry Song <baohua@xxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
> Cc: Usama Arif <usamaarif642@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
>  Documentation/filesystems/proc.rst | 37 ++++++++++++++++++++++--------
>  1 file changed, 27 insertions(+), 10 deletions(-)
>
LGTM.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi