Re: [PATCH mm-unstable v19 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support

[Zi Yan]

On 5 Jun 2026, at 12:14, Nico Pache wrote:

> generalize the order of the __collapse_huge_page_* and collapse_max_*
> functions to support future mTHP collapse.
>
> The current mechanism for determining collapse with the
> khugepaged_max_ptes_none value is not designed with mTHP in mind. This
> raises a key design issue: if we support user defined max_pte_none values
> (even those scaled by order), a collapse of a lower order can introduces
> an feedback loop, or "creep", when max_ptes_none is set to a value greater
> than HPAGE_PMD_NR / 2. [1]
>
> With this configuration, a successful collapse to order N will populate
> enough pages to satisfy the collapse condition on order N+1 on the next
> scan. This leads to unnecessary work and memory churn.
>
> To fix this issue introduce a helper function that will limit mTHP
> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> This effectively supports two modes: [2]
>
> - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE
>   that maps the shared zeropage. Consequently, no memory bloat.
> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>   available mTHP order.
>
> This removes the possibility of "creep", and a warning will be emitted if
> any non-supported max_ptes_none value is configured with mTHP enabled.
> Any intermediate value will default mTHP collapse to max_ptes_none=0.
>
> mTHP collapse will not honor the khugepaged_max_ptes_shared or
> khugepaged_max_ptes_swap parameters, and will fail if it encounters a
> shared or swapped entry.
>
> No functional changes in this patch; however it defines future behavior
> for mTHP collapse.
>
> [1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@xxxxxxx
> [2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@xxxxxxxxxx
>
> Reviewed-by: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> Acked-by: David Hildenbrand (arm) <david@xxxxxxxxxx>
> Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> Co-developed-by: Dev Jain <dev.jain@xxxxxxx>
> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> Signed-off-by: Nico Pache <npache@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 126 +++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 93 insertions(+), 33 deletions(-)
>
LGTM.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi