linux-kernel - Re: [PATCH 1/3] kvfree_rcu: Allocate a page for a single argument

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Message-ID: <20210125132236.GJ827@dhcp22.suse.cz>
Date:   Mon, 25 Jan 2021 14:22:36 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
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>,
        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 1/3] kvfree_rcu: Allocate a page for a single argument

On Wed 20-01-21 17:21:46, Uladzislau Rezki (Sony) wrote:
> For a single argument we can directly request a page from a caller
> context when a "carry page block" is run out of free spots. Instead
> of hitting a slow path we can request an extra page by demand and
> proceed with a fast path.
> 
> A single-argument kvfree_rcu() must be invoked in sleepable contexts,
> and that its fallback is the relatively high latency synchronize_rcu().
> Single-argument kvfree_rcu() therefore uses GFP_KERNEL|__GFP_RETRY_MAYFAIL
> to allow limited sleeping within the memory allocator.

__GFP_RETRY_MAYFAIL can be quite heavy. It is effectively the most heavy
way to allocate without triggering the OOM killer. Is this really what
you need/want? Is __GFP_NORETRY too weak?

-- 
Michal Hocko
SUSE Labs