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Date:   Thu, 24 Sep 2020 13:16:32 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Uladzislau Rezki <urezki@gmail.com>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.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>,
        Mel Gorman <mgorman@suse.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Message-ID: <20200924111632.GD2628@hirez.programming.kicks-ass.net>
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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.

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.

The preemption context does not say antying much useful about the
allocation context.