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

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Message-ID: <20200930092732.GP2277@dhcp22.suse.cz>
Date:   Wed, 30 Sep 2020 11:27:32 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        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>,
        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 Tue 29-09-20 18:25:14, Uladzislau Rezki wrote:
> > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability
> > > to provide a memory service for contexts which are not allowed to
> > > sleep, RCU is part of them. Both flags used to provide such ability
> > > before but not anymore.
> > > 
> > > Do you agree with it?
> > 
> > Yes this sucks. But this is something that we likely really want to live
> > with. We have to explicitly _document_ that really atomic contexts in RT
> > cannot use the allocator. From the past discussions we've had this is
> > likely the most reasonable way forward because we do not really want to
> > encourage anybody to do something like that and there should be ways
> > around that. The same is btw. true also for !RT. The allocator is not
> > NMI safe and while we should be able to make it compatible I am not
> > convinced we really want to.
> > 
> > Would something like this be helpful wrt documentation?
> > 
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index 67a0774e080b..9fcd47606493 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -238,7 +238,9 @@ struct vm_area_struct;
> >   * %__GFP_FOO flags as necessary.
> >   *
> >   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
> > - * watermark is applied to allow access to "atomic reserves"
> > + * watermark is applied to allow access to "atomic reserves".
> > + * The current implementation doesn't support NMI and other non-preemptive context
> > + * (e.g. raw_spin_lock).
> >   *
> >   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
> >   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> > 
> To me it is clear. But also above conflicting statement:
> 
> <snip>
> %GFP_ATOMIC users can not sleep and need the allocation to succeed. A %lower
> <snip>
> 
> should be rephrased, IMHO.

Any suggestions? Or more specifics about which part is conflicting? It
tries to say that there is a higher demand to succeed even though the
context cannot sleep to take active measures to achieve that. So the
only way to achieve that is to break the watermakrs to a certain degree
which is making them more "higher class" than other allocations.

-- 
Michal Hocko
SUSE Labs