[PATCH bpf-next v3 4/6] memcg: Use trylock to access memcg stock_lock.

[alexei.starovoitov@gmail.com]

From: Alexei Starovoitov <ast@kernel.org>

Teach memcg to operate under trylock conditions when
spinning locks cannot be used.
The end result is __memcg_kmem_charge_page() and
__memcg_kmem_uncharge_page() are safe to use from
any context in RT and !RT.
In !RT the NMI handler may fail to trylock stock_lock.
In RT hard IRQ and NMI handlers will not attempt to trylock.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
 mm/memcontrol.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7b3503d12aaf..f168d223375f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1756,7 +1756,8 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock,
  *
  * returns true if successful, false otherwise.
  */
-static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
+static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages,
+			  gfp_t gfp_mask)
 {
 	struct memcg_stock_pcp *stock;
 	unsigned int stock_pages;
@@ -1766,7 +1767,11 @@ static bool consume_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 	if (nr_pages > MEMCG_CHARGE_BATCH)
 		return ret;
 
-	local_lock_irqsave(&memcg_stock.stock_lock, flags);
+	if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
+		if (gfp_mask & __GFP_TRYLOCK)
+			return ret;
+		local_lock_irqsave(&memcg_stock.stock_lock, flags);
+	}
 
 	stock = this_cpu_ptr(&memcg_stock);
 	stock_pages = READ_ONCE(stock->nr_pages);
@@ -1851,7 +1856,14 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
 {
 	unsigned long flags;
 
-	local_lock_irqsave(&memcg_stock.stock_lock, flags);
+	if (!local_trylock_irqsave(&memcg_stock.stock_lock, flags)) {
+		/*
+		 * In case of unlikely failure to lock percpu stock_lock
+		 * uncharge memcg directly.
+		 */
+		mem_cgroup_cancel_charge(memcg, nr_pages);
+		return;
+	}
 	__refill_stock(memcg, nr_pages);
 	local_unlock_irqrestore(&memcg_stock.stock_lock, flags);
 }
@@ -2196,7 +2208,7 @@ int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	unsigned long pflags;
 
 retry:
-	if (consume_stock(memcg, nr_pages))
+	if (consume_stock(memcg, nr_pages, gfp_mask))
 		return 0;
 
 	if (!do_memsw_account() ||
-- 
2.43.5