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

["Ritesh Harjani (IBM)" <ritesh.list@gmail.com>]

On PowerPC Book3S64, MMU is selected at runtime, so macros like
PMD_SHIFT are effectively runtime variables in the Book3S64 code. THP
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;
+
 /* May return NULL on invalid type, caller must check for NULL return */
 static struct swap_info_struct *swap_type_to_info(int type)
 {
@@ -437,7 +439,7 @@ static void swap_cluster_free_table(struct swap_cluster_info *ci)
 		return;

 	rcu_assign_pointer(ci->table, NULL);
-	if (!SWP_TABLE_USE_PAGE) {
+	if (!swap_table_use_page) {
 		kmem_cache_free(swap_table_cachep, table);
 		return;
 	}
@@ -456,7 +458,7 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp)
 	if (rcu_access_pointer(ci->table))
 		return 0;

-	if (SWP_TABLE_USE_PAGE) {
+	if (swap_table_use_page) {
 		folio = folio_alloc(gfp | __GFP_ZERO, 0);
 		if (folio)
 			table = folio_address(folio);
@@ -471,7 +473,8 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp)
 #ifdef CONFIG_MEMCG
 	if (!mem_cgroup_disabled()) {
 		VM_WARN_ON_ONCE(ci->memcg_table);
-		ci->memcg_table = kzalloc_obj(*ci->memcg_table, gfp);
+		ci->memcg_table = kzalloc_flex(*ci->memcg_table, id,
+					       SWAPFILE_CLUSTER, gfp);
 		if (!ci->memcg_table) {
 			swap_cluster_free_table(ci);
 			return -ENOMEM;
@@ -3912,14 +3915,18 @@ static int __init swapfile_init(void)
 {
 	swapfile_maximum_size = arch_max_swapfile_size();

+	swap_table_use_page =
+		(SWAPFILE_CLUSTER * sizeof(atomic_long_t) == PAGE_SIZE);
+
 	/*
 	 * Once a cluster is freed, it's swap table content is read
 	 * only, and all swap cache readers (swap_cache_*) verifies
 	 * the content before use. So it's safe to use RCU slab here.
 	 */
-	if (!SWP_TABLE_USE_PAGE)
+	if (!swap_table_use_page)
 		swap_table_cachep = kmem_cache_create("swap_table",
-				    sizeof(struct swap_table),
+				    struct_size_t(struct swap_table, entries,
+					    SWAPFILE_CLUSTER),
 				    0, SLAB_PANIC | SLAB_TYPESAFE_BY_RCU, NULL);

 #ifdef CONFIG_MIGRATION
--
2.39.5