linux-kernel - Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page

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Message-ID: <20200921074716.GC12990@dhcp22.suse.cz>
Date:   Mon, 21 Sep 2020 09:47:16 +0200
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>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

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