Re: [PATCH v4 1/3] mm, swap: make SWAPFILE_CLUSTER runtime

[Barry Song]

On Fri, Jun 19, 2026 at 12:41 PM Ritesh Harjani (IBM)
<ritesh.list@xxxxxxxxx> wrote:
>
> On PowerPC Book3S64, MMU is selected at runtime, so macros like
> PMD_SHIFT are effectively runtime variables in the Book3S64 code. THP

Not an expert on Book3S64—could you explain the runtime variables in
more detail? Does enabling THP_SWAP on PowerPC cause any build issues?

> swap code uses these macros to size some of its array data structures
> based on PMD_ORDER e.g. SWAPFILE_CLUSTER macro is used for this very
> purpose.
> Hence this patch makes the users of SWAPFILE_CLUSTER to use this macro value at
> runtime and also modifies swap_table and swap_memcg_table which were earlier
> using this macro for defining the number of table entries.
>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> ---
>  mm/swap_table.h |  6 ++----
>  mm/swapfile.c   | 17 ++++++++++++-----
>  2 files changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/mm/swap_table.h b/mm/swap_table.h
> index e6613e62f8d0..90e2a7852300 100644
> --- a/mm/swap_table.h
> +++ b/mm/swap_table.h
> @@ -8,16 +8,14 @@
>
>  /* A typical flat array in each cluster as swap table */
>  struct swap_table {
> -       atomic_long_t entries[SWAPFILE_CLUSTER];
> +       DECLARE_FLEX_ARRAY(atomic_long_t, entries);
>  };
>
>  /* For storing memcg private id */
>  struct swap_memcg_table {
> -       unsigned short id[SWAPFILE_CLUSTER];
> +       DECLARE_FLEX_ARRAY(unsigned short, id);
>  };
>
> -#define SWP_TABLE_USE_PAGE (sizeof(struct swap_table) == PAGE_SIZE)
> -
>  /*
>   * A swap table entry represents the status of a swap slot on a swap
>   * (physical or virtual) device. The swap table in each cluster is a
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 78b49b0658ad..4bf11c5b87eb 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -129,6 +129,8 @@ static DEFINE_PER_CPU(struct percpu_swap_cluster, percpu_swap_cluster) = {
>         .lock = INIT_LOCAL_LOCK(),
>  };
>
> +static bool swap_table_use_page __ro_after_init;

Does a static key help here?

Best Regards
Barry