linux-kernel - [tip: core/rcu] kvfree_rcu: Replace __GFP_RETRY_MAYFAIL by __GFP

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Date:   Sun, 11 Apr 2021 13:43:40 -0000
From:   "tip-bot2 for Uladzislau Rezki (Sony)" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Uladzislau Rezki (Sony)" <urezki@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: core/rcu] kvfree_rcu: Replace __GFP_RETRY_MAYFAIL by __GFP_NORETRY

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     3e7ce7a187fc6aaa9fda1310a2b8da8770342ff7
Gitweb:        https://git.kernel.org/tip/3e7ce7a187fc6aaa9fda1310a2b8da8770342ff7
Author:        Uladzislau Rezki (Sony) <urezki@...il.com>
AuthorDate:    Fri, 29 Jan 2021 17:16:03 +01:00
Committer:     Paul E. McKenney <paulmck@...nel.org>
CommitterDate: Mon, 08 Mar 2021 14:18:07 -08:00

kvfree_rcu: Replace __GFP_RETRY_MAYFAIL by __GFP_NORETRY

__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@...il.com>
Acked-by: Michal Hocko <mhocko@...e.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 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);
 		}