[PATCH v2 0/3] mm, swap: Enable THP SWAP for PowerPC Book3S64

[PATCH v2 0/3] mm, swap: Enable THP SWAP for PowerPC Book3S64

[Ritesh Harjani (IBM)]

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 for e.g. to size some of its array data structures based on
PMD_ORDER. This patch series makes that usage dependent on the runtime
variable and provides an upper-bound architecture override for cases (e.g.
SWAP_NR_ORDERS), where the runtime conversion is not considered beneficial.

This series increases bandwidth throughput with zram backend for swapout by
around 40-50% with Radix and 100-130% with Hash (Tested by Sayali)

Note that this patch series is based out of linux-next (next-20260608).

RFC -> v2:
==========
1. Send the unused leftovers change in swap.h separately [1]
2. Changed RFC Patch-3 design from runtime SWAP_NR_ORDERS to arch override
   (ARCH_MAX_PMD_ORDER) - suggested by Youngjun
3. Dropped RFC tag

[1]: https://lore.kernel.org/linux-mm/68591daf0d679e5a0072d63751f187d14613e2b0.1781146877.git.ritesh.list@gmail.com/
[RFC]: https://lore.kernel.org/linux-mm/cover.1781000840.git.ritesh.list@gmail.com/

Ritesh Harjani (IBM) (3):
  mm, swap: make SWAPFILE_CLUSTER runtime
  mm, swap: allow archs to override SWAP_NR_ORDERS via ARCH_MAX_PMD_ORDER
  powerpc: Kconfig: Enable THP_SWAP on Book3S64

 arch/powerpc/include/asm/book3s/64/pgtable.h |  7 +++++
 arch/powerpc/platforms/Kconfig.cputype       |  1 +
 include/linux/swap.h                         | 12 ++++++++-
 mm/swap.h                                    |  5 ++--
 mm/swap_table.h                              |  6 ++---
 mm/swapfile.c                                | 27 ++++++++++++++++----
 6 files changed, 46 insertions(+), 12 deletions(-)

--
2.39.5

[PATCH v2 1/3] mm, swap: make SWAPFILE_CLUSTER runtime

[Ritesh Harjani (IBM)]

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 initializes SWAPFILE_CLUSTER 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.h       |  5 +++--
 mm/swap_table.h |  6 ++----
 mm/swapfile.c   | 27 ++++++++++++++++++++++-----
 3 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/mm/swap.h b/mm/swap.h
index 77d2d14eda42..956879a69ddd 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -26,11 +26,12 @@ extern int page_cluster;
 #define SWAP_TABLE_HAS_ZEROFLAG		((BITS_PER_LONG - SWAP_CACHE_PFN_MARK_BITS - \
 					  SWAP_CACHE_PFN_BITS) > SWAP_COUNT_MIN_BITS)
 
+extern unsigned int swap_slots_in_cluster __read_mostly;
+#define SWAPFILE_CLUSTER	swap_slots_in_cluster
+
 #ifdef CONFIG_THP_SWAP
-#define SWAPFILE_CLUSTER	HPAGE_PMD_NR
 #define swap_entry_order(order)	(order)
 #else
-#define SWAPFILE_CLUSTER	256
 #define swap_entry_order(order)	0
 #endif
 
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..016a5aa0cb93 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -129,6 +129,17 @@ static DEFINE_PER_CPU(struct percpu_swap_cluster, percpu_swap_cluster) = {
 	.lock = INIT_LOCAL_LOCK(),
 };
 
+unsigned int swap_slots_in_cluster __read_mostly;
+bool swap_table_use_page __read_mostly;
+
+static unsigned int generic_swap_slots_in_clusters(void)
+{
+	if (IS_ENABLED(CONFIG_THP_SWAP))
+		return HPAGE_PMD_NR;
+	else
+		return 256;
+}
+
 /* 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 +448,7 @@ 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 +467,7 @@ 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 +482,8 @@ 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 +3924,19 @@ static int __init swapfile_init(void)
 {
 	swapfile_maximum_size = arch_max_swapfile_size();
 
+	swap_slots_in_cluster = generic_swap_slots_in_clusters();
+	swap_table_use_page =
+		(swap_slots_in_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

[PATCH v2 2/3] mm, swap: allow archs to override SWAP_NR_ORDERS via ARCH_MAX_PMD_ORDER

[Ritesh Harjani (IBM)]

SWAP_NR_ORDERS sizes a few small bounded arrays inside THP swap
allocator code (nofull/frag cluster lists, percpu_swap_cluster's
si/offset arrays, next array for rotational device). This currently
expands to PMD_ORDER+1, which only works when PMD_ORDER is a compile
time constant.

However on architecture like PowerPC Book3S64, PMD_ORDER is a runtime
variable which depends upon which MMU is selected (Radix / Hash), so in
that case, PMD_ORDER cannot be used to size the static arrays.

This patch provides an optional ARCH_MAX_PMD_ORDER (upper-bound)
override for such architectures. The memory overhead on enabling this
override is negligible. Even if we make SWAP_NR_ORDERS runtime alloc,
default slab padding could cause some memory waste. Also we lose the
per-cpu cacheline benefits (for percpu_swap_cluster) because it might
cost an extra cacheline indirection overhead in swap_alloc_fast() for
fetching si[order]/offset[order]. Note that a fully runtime
SWAP_NR_ORDERS was considered in previous version but was dropped for
this reason [1]

[1]: https://lore.kernel.org/linuxppc-dev/pl1zdksc.ritesh.list@gmail.com/

Suggested-by: YoungJun Park <youngjun.park@lge.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 arch/powerpc/include/asm/book3s/64/pgtable.h |  7 +++++++
 include/linux/swap.h                         | 12 +++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
index e67e64ac6e8c..7f22d5d5fbdf 100644
--- a/arch/powerpc/include/asm/book3s/64/pgtable.h
+++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
@@ -204,6 +204,13 @@ extern unsigned long __pmd_frag_size_shift;
 #define MAX_PTRS_PER_PGD	(1 << (H_PGD_INDEX_SIZE > RADIX_PGD_INDEX_SIZE ? \
 				       H_PGD_INDEX_SIZE : RADIX_PGD_INDEX_SIZE))
 
+/*
+ * Compile-time upper bound on PMD_ORDER across hash and radix MMUs.
+ * Used by THP SWAP code. Check include/linux/swap.h
+ */
+#define ARCH_MAX_PMD_ORDER ((H_PTE_INDEX_SIZE > RADIX_PTE_INDEX_SIZE) ? \
+				H_PTE_INDEX_SIZE : RADIX_PTE_INDEX_SIZE)
+
 /* PMD_SHIFT determines what a second-level page table entry can map */
 #define PMD_SHIFT	(PAGE_SHIFT + PTE_INDEX_SIZE)
 #define PMD_SIZE	(1UL << PMD_SHIFT)
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 46c25523d7b8..4e1701b4a565 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -223,11 +223,21 @@ enum {
  */
 #define SWAP_ENTRY_INVALID	0
 
+/*
+ * ARCH_MAX_PMD_ORDER is an optional arch hook: a compile-time upper bound for
+ * PMD_ORDER across all possible MMU configurations of that arch. It is used to
+ * size SWAP_NR_ORDERS on architectures (e.g. powerpc book3s64) where PMD_ORDER
+ * is selected at boot rather than at compile time.
+ */
 #ifdef CONFIG_THP_SWAP
+#ifdef ARCH_MAX_PMD_ORDER
+#define SWAP_NR_ORDERS		(ARCH_MAX_PMD_ORDER + 1)
+#else
 #define SWAP_NR_ORDERS		(PMD_ORDER + 1)
+#endif /* ARCH_MAX_PMD_ORDER */
 #else
 #define SWAP_NR_ORDERS		1
-#endif
+#endif /* CONFIG_THP_SWAP */
 
 /*
  * We keep using same cluster for rotational device so IO will be sequential.
-- 
2.39.5

[PATCH v2 3/3] powerpc: Kconfig: Enable THP_SWAP on Book3S64

[Ritesh Harjani (IBM)]

THP_SWAP avoids splitting of a transparent huge folio into 32 smaller
64K folios (Radix-64K pagesize / 2M PMD) or into 256 smaller 64K folios
(Hash-64K pagesize / 16M PMD), during swapout. This improves the
swapping performance since all the bookking & I/O submission happens
once per large folio. More details at [1].

PowerPC Book3S64 could not enable this before because PMD_ORDER is
selected at runtime depending upon the chosen MMU. The earlier patches
in this series turn SWAPFILE_CLUSTER into a runtime value and introduce
an ARCH_MAX_PMD_ORDER upperbound override for SWAP_NR_ORDERS. With those
changes, we can now enable THP SWAP for Book3S64.

This increases bandwidth throughput with zram backend for swapout by
40-50% with Radix and 100-130% with Hash (Tested by Sayali)

[1]: https://lore.kernel.org/all/20170515112522.32457-2-ying.huang@intel.com/

Tested-by: Sayali Patil <sayalip@linux.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
---
 arch/powerpc/platforms/Kconfig.cputype | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index bac02c83bb3e..48f74bd22343 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -113,6 +113,7 @@ config PPC_THP
        select HAVE_ARCH_TRANSPARENT_HUGEPAGE
        select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
        select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
+       select ARCH_WANTS_THP_SWAP	if TRANSPARENT_HUGEPAGE

 choice
 	prompt "CPU selection"
--
2.39.5