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

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Date:   Wed, 30 Sep 2020 13:48:40 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        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 Wed, Sep 30, 2020 at 1:22 PM Michal Hocko <mhocko@...e.com> wrote:

> > > > I think documenting is useful.
> > > >
> > > > Could it be more explicit in what the issue is? Something like:
> > > >
> > > > * Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT
> > > > systems. Therefore, the current low-level allocator implementation does not
> > > > support being called from special contexts that are atomic on RT - such as
> > > > NMI and raw_spin_lock. Due to these constraints and considering calling code
> > > > usually has no control over the PREEMPT_RT configuration, callers of the
> > > > allocator should avoid calling the allocator from these cotnexts even in
> > > > non-RT systems.
> > >
> > > I do not mind documenting RT specific behavior but as mentioned in other
> > > reply, this should likely go via RT tree for now. There is likely more
> > > to clarify about atomicity for PREEMPT_RT.
> >
> > I am sorry, I did not understand what you meant by something missing
> > in Linus Tree. CONFIG_PREEMPT_RT is there.
>
> OK, I was not aware we already CONFIG_PREEMPT_RT in the three. There is
> still a lot from the RT patchset including sleeping spin locks that make
> a real difference. Or maybe I haven't checked properly.
>
> > Could you clarify? Also the CONFIG_PREEMPT_RT is the only thing
> > driving this requirement for GFP_ATOMIC right? Or are there non-RT
> > reasons why GFP_ATOMIC allocation cannot be done from
> > NMI/raw_spin_lock ?
>
> I have already sent a clarification patch [1]. NMI is not supported
> regardless of the preemption mode. Hope this clarifies it a bit.
>
> [1] http://lkml.kernel.org/r/20200929123010.5137-1-mhocko@kernel.org

That works for me. Thanks,

  - Joel