Barry Song [off-list ref] writes:

On Fri, Jun 19, 2026 at 12:41 PM Ritesh Harjani (IBM)
[off-list ref] wrote:

quoted

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?

yes, build issues. We cannot declare array sizes by using runtime
variables. That's what this patch series fixes.

quoted

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@gmail.com>
---
 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?

That IMO won't give much benefit, given the allocation by either kmem or
alloc pages, anyway dominates the cost. Also I believe this is exactly
the usecase where branch predictor helps signficantly and reliably given
the variable is ro_after_init.

Best Regards
Barry

Thanks Barry for looking into this.

-ritesh