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Date:   Mon, 21 Sep 2020 09:47:16 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        Joel Fernandes <joel@joelfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Message-ID: <20200921074716.GC12990@dhcp22.suse.cz>
References: <20200918194817.48921-1-urezki@gmail.com>
 <20200918194817.48921-3-urezki@gmail.com>
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On Fri 18-09-20 21:48:15, Uladzislau Rezki (Sony) wrote:
[...]
> Proposal
> ========
> Introduce a lock-free function that obtain a page from the per-cpu-lists
> on current CPU. It returns NULL rather than acquiring any non-raw spinlock.

I was not happy about this solution when we have discussed this
last time and I have to say I am still not happy. This is exposing
an internal allocator optimization and allows a hard to estimate
consumption of pcp free pages. IIUC this run on pcp cache can be
controled directly from userspace (close(open) loop IIRC) which makes it
even bigger no-no.

I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@nanos.tec.linutronix.de
that this optimization is not aiming at reasonable workloads. Really, go
with pre-allocated buffer and fallback to whatever slow path you have
already. Exposing more internals of the allocator is not going to do any
good for long term maintainability.
-- 
Michal Hocko
SUSE Labs