Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 30 Sep 2020 11:25:17 -0400
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Theodore Y . Ts'o" <tytso@mit.edu>,
        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: <20200930152517.GA1470428@google.com>
References: <20200921194819.GA24236@pc636>
 <20200922075002.GU12990@dhcp22.suse.cz>
 <20200922131257.GA29241@pc636>
 <20200923103706.GJ3179@techsingularity.net>
 <20200923154105.GO29330@paulmck-ThinkPad-P72>
 <20200923232251.GK3179@techsingularity.net>
 <20200924081614.GA14819@pc636>
 <20200925080503.GC3389@dhcp22.suse.cz>
 <20200925153129.GB25350@pc636>
 <20200925154741.GI3389@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200925154741.GI3389@dhcp22.suse.cz>
Precedence: bulk

On Fri, Sep 25, 2020 at 05:47:41PM +0200, Michal Hocko wrote:
> On Fri 25-09-20 17:31:29, Uladzislau Rezki wrote:
> > > > > > 
> > > > > > All good points!
> > > > > > 
> > > > > > On the other hand, duplicating a portion of the allocator functionality
> > > > > > within RCU increases the amount of reserved memory, and needlessly most
> > > > > > of the time.
> > > > > > 
> > > > > 
> > > > > But it's very similar to what mempools are for.
> > > > > 
> > > > As for dynamic caching or mempools. It requires extra logic on top of RCU
> > > > to move things forward and it might be not efficient way. As a side
> > > > effect, maintaining of the bulk arrays in the separate worker thread
> > > > will introduce other drawbacks:
> > > 
> > > This is true but it is also true that it is RCU to require this special
> > > logic and we can expect that we might need to fine tune this logic
> > > depending on the RCU usage. We definitely do not want to tune the
> > > generic page allocator for a very specific usecase, do we?
> > > 
> > 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).

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.

thanks!

 - Joel


>   *
>   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
>   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> 
> [...]
> -- 
> Michal Hocko
> SUSE Labs