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

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Date:   Thu, 24 Sep 2020 17:16:25 +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 Thu, Sep 24, 2020 at 01:16:32PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 24, 2020 at 10:16:14AM +0200, Uladzislau Rezki wrote:
> > Other option is if we had unconditionally enabled PREEMPT_COUNT config.
> > It would be easy to identify a context type and invoke a page allocator
> > if a context is preemtale. But as of now preemptable() is "half" working.
> > Thomas uploaded patches to make it unconditional. But it can be blocked.
> 
> While I in principle support Thomas' patch, I think this is an abuse and
> still complete wrong.
> 
Good that you support it :)

>
> Not all preemptible() context can deal with GFP_KERNEL (GFP_NOFS
> existing should be a clue). !preemptible context cannot unconditionally
> deal with GFP_NOWAIT, consider using it while holding zone->lock.
> 
Not sure if i fully follow you here. What i tried to express, if we had
preemtable() fully working, including !PREEMPT, we can at least detect
the following:

if (!preemptable())
    by pass using any ATOMIC/NOWAIT flags because they do not work;

as an example you mentioned z->lock that is sleepable.

>
> The preemption context does not say antying much useful about the
> allocation context.
>
I am talking about taking a decision. 

--
Vlad Rezki