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

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Message-ID: <20200925102531.GA25350@pc636>
Date:   Fri, 25 Sep 2020 12:25:31 +0200
From:   Uladzislau Rezki <urezki@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        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>,
        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>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

On Fri, Sep 25, 2020 at 10:15:52AM +0200, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 05:21:12PM +0200, Uladzislau Rezki wrote:
> > On Thu, Sep 24, 2020 at 01:19:07PM +0200, Peter Zijlstra wrote:
> > > On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
> > > > The key point is "enough". We need pages to make a) fast progress b) support
> > > > single argument of kvfree_rcu(one_arg). Not vice versa. That "enough" depends
> > > > on scheduler latency and vague pre-allocated number of pages, it might
> > > > be not enough what would require to refill it more and more or we can overshoot
> > > > that would lead to memory overhead. So we have here timing issues and
> > > > not accurate model. IMHO.
> > > 
> > > I'm firmly opposed to the single argument kvfree_rcu() idea, that's
> > > requiring memory to free memory.
> > > 
> > Hmm.. The problem is there is a demand in it:
> 
> People demand ponies all the time, the usual answer is: No.
>
I see your view. From the other hand it is clear, there is still
demand in it:

<snip>
void ext4_kvfree_array_rcu(void *to_free)
{
 struct ext4_rcu_ptr *ptr = kzalloc(sizeof(*ptr), GFP_KERNEL);

 if (ptr) {
  ptr->ptr = to_free;
  call_rcu(&ptr->rcu, ext4_rcu_ptr_callback);
  return;
 }
 synchronize_rcu();
 kvfree(ptr);
}
<snip>

--
Vlad Rezki