linux-kernel - Re: [PATCH 2/3] kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument kvfree

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Date:   Thu, 28 Jan 2021 19:06:58 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 2/3] kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument
 kvfree_rcu()

On Wed, Jan 20, 2021 at 05:21:47PM +0100, Uladzislau Rezki (Sony) wrote:
> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> This commit applies the __GFP_NOMEMALLOC gfp flag to memory allocations
> carried out by the single-argument variant of kvfree_rcu(), thus avoiding
> this can-sleep code path from dipping into the emergency reserves.
> 
> Acked-by: Michal Hocko <mhocko@...e.com>
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 2014fb22644d..454809514c91 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3491,7 +3491,7 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
>  		if (!bnode && can_alloc) {
>  			krc_this_cpu_unlock(*krcp, *flags);
>  			bnode = (struct kvfree_rcu_bulk_data *)
> -				__get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
> +				__get_free_page(GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOMEMALLOC | __GFP_NOWARN);
>  			*krcp = krc_this_cpu_lock(flags);
>  		}
>  
> -- 
> 2.20.1
> 
Please see below a V2:

V1 -> V2:
    - rebase on [PATCH v2 1/1] kvfree_rcu: Directly allocate page for single-argument
    - add a comment about __GFP_NOMEMALLOC usage.

<snip>
>From 1427698cdbdced53d9b5eee60aa5d22bc223056d Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@...nel.org>
Date: Wed, 20 Jan 2021 17:21:47 +0100
Subject: [PATCH v2 1/1] kvfree_rcu: Use __GFP_NOMEMALLOC for single-argument
 kvfree_rcu()

This commit applies the __GFP_NOMEMALLOC gfp flag to memory allocations
carried out by the single-argument variant of kvfree_rcu(), thus avoiding
this can-sleep code path from dipping into the emergency reserves.

Acked-by: Michal Hocko <mhocko@...e.com>
Suggested-by: Michal Hocko <mhocko@...e.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e450c17a06b2..e7b705155c92 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3496,11 +3496,14 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
 			// 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_NORETRY | __GFP_NOWARN);
+				__get_free_page(GFP_KERNEL | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN);
 			*krcp = krc_this_cpu_lock(flags);
 		}
 
-- 
2.20.1
<snip>

Thanks!

--
Vlad Rezki