[PATCH tip/core/rcu 4/6] kvfree_rcu: Replace __GFP_RETRY_MAYFAIL by __GFP_NORETRY

[paulmck]

From: "Uladzislau Rezki (Sony)" <urezki@xxxxxxxxx>

__GFP_RETRY_MAYFAIL can spend quite a bit of time reclaiming, and this
can be wasted effort given that there is a fallback code path in case
memory allocation fails.

__GFP_NORETRY does perform some light-weight reclaim, but it will fail
under OOM conditions, allowing the fallback to be taken as an alternative
to hard-OOMing the system.

There is a four-way tradeoff that must be balanced:
    1) Minimize use of the fallback path;
    2) Avoid full-up OOM;
    3) Do a light-wait allocation request;
    4) Avoid dipping into the emergency reserves.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7ee83f3..0ecc1fb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3517,8 +3517,20 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
 		bnode = get_cached_bnode(*krcp);
 		if (!bnode && can_alloc) {
 			krc_this_cpu_unlock(*krcp, *flags);
+
+			// __GFP_NORETRY - allows a light-weight direct reclaim
+			// what is OK from minimizing of fallback hitting point of
+			// view. Apart of that it forbids any OOM invoking what is
+			// also beneficial since we are about to release memory soon.
+			//
+			// __GFP_NOMEMALLOC - prevents from consuming of all the
+			// memory reserves. Please note we have a fallback path.
+			//
+			// __GFP_NOWARN - it is supposed that an allocation can
+			// be failed under low memory or high memory pressure
+			// scenarios.
 			bnode = (struct kvfree_rcu_bulk_data *)
-				__get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
+				__get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
 			*krcp = krc_this_cpu_lock(flags);
 		}
 
-- 
2.9.5