[PATCH v2 3/4] memcg: int16_t for cached slab stats

[Shakeel Butt <shakeel.butt@linux.dev>]

Currently struct obj_stock_pcp stores cached slab stats in 'int' which
is 4 bytes per counter on 64-bit machines. Switch them to int16_t to
shrink the cached metadata.

The existing PAGE_SIZE flush in __account_obj_stock() bounds *bytes at
PAGE_SIZE on 4KiB and 16KiB page archs, well within int16_t. On 64KiB
pages PAGE_SIZE is well above S16_MAX so that flush never fires, and a
sufficiently long run of accumulations would overflow the cache. Add
an explicit S16_MAX guard before each add: when the next add would
push abs(*bytes) past S16_MAX, fold the cached value into @nr and
flush directly via mod_objcg_mlstate() before the accumulation.

Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
---

Changes since v2:
- Collected tags

 mm/memcontrol.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e4f00a8159d5..78c02451312b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2022,8 +2022,8 @@ struct obj_stock_pcp {
 	struct obj_cgroup *cached_objcg;
 	uint16_t nr_bytes;
 	int16_t node_id;
-	int nr_slab_reclaimable_b;
-	int nr_slab_unreclaimable_b;
+	int16_t nr_slab_reclaimable_b;
+	int16_t nr_slab_unreclaimable_b;
 
 	struct work_struct work;
 	unsigned long flags;
@@ -3158,7 +3158,7 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 				struct obj_stock_pcp *stock, int nr,
 				struct pglist_data *pgdat, enum node_stat_item idx)
 {
-	int *bytes;
+	int16_t *bytes;
 
 	/*
 	 * Though at the moment MAX_NUMNODES <= 1024 in all archs but let's make
@@ -3195,6 +3195,16 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 
 	bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
 					       : &stock->nr_slab_unreclaimable_b;
+	/*
+	 * To avoid overflow or underflow, flush directly if accumulating @nr
+	 * would push the cached value past S16_MAX.
+	 */
+	if (abs(nr + *bytes) > S16_MAX) {
+		nr += *bytes;
+		*bytes = 0;
+		goto direct;
+	}
+
 	/*
 	 * Even for large object >= PAGE_SIZE, the vmstat data will still be
 	 * cached locally at least once before pushing it out.
-- 
2.53.0-Meta