linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Tue, 18 Aug 2020 09:43:44 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "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 1/2] mm: Add __GFP_NO_LOCKS flag

On Mon 17-08-20 15:28:03, Paul E. McKenney wrote:
> On Mon, Aug 17, 2020 at 10:28:49AM +0200, Michal Hocko wrote:
> > On Mon 17-08-20 00:56:55, Uladzislau Rezki wrote:
> 
> [ . . . ]
> 
> > > wget ftp://vps418301.ovh.net/incoming/1000000_kmalloc_kfree_rcu_proc_percpu_pagelist_fractio_is_8.png
> > 
> > 1/8 of the memory in pcp lists is quite large and likely not something
> > used very often.
> > 
> > Both these numbers just make me think that a dedicated pool of page
> > pre-allocated for RCU specifically might be a better solution. I still
> > haven't read through that branch of the email thread though so there
> > might be some pretty convincing argments to not do that.
> 
> To avoid the problematic corner cases, we would need way more dedicated
> memory than is reasonable, as in well over one hundred pages per CPU.
> Sure, we could choose a smaller number, but then we are failing to defend
> against flooding, even on systems that have more than enough free memory
> to be able to do so.  It would be better to live within what is available,
> taking the performance/robustness hit only if there isn't enough.

Thomas had a good point that it doesn't really make much sense to
optimize for flooders because that just makes them more effective.

> My current belief is that we need a combination of (1) either the
> GFP_NOLOCK flag or Peter Zijlstra's patch and

I must have missed the patch?
-- 
Michal Hocko
SUSE Labs