linux-kernel - Re: [PATCH 19/24] rcu/tree: Support reclaim for head-less object

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Date:   Mon, 4 May 2020 10:08:33 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        RCU <rcu@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 19/24] rcu/tree: Support reclaim for head-less object

On Mon, May 04, 2020 at 06:56:29PM +0200, Uladzislau Rezki wrote:
> > > 
> > > For single argument we can drop the lock before the entry to the page
> > > allocator. Because it follows might_sleep() anotation we avoid of having
> > > a situation when spinlock(rt mutex) is taken from any atomic context.
> > > 
> > > Since the lock is dropped the current context can be interrupted by
> > > an IRQ which in its turn can also call kvfree_rcu() on current CPU.
> > > In that case it must be double argument(single is not allowed) kvfree_rcu()
> > > call. For PREEMPT_RT if no cache everything is reverted to rcu_head usage,
> > > i.e. the entry to page allocator is bypassed.
> > > 
> > > It can be addressed as a separate patch and send out later on if we
> > > are on the same page.
> > > 
> > > Paul, Joel what are your opinions?
> > 
> > I strongly prefer that it be removed from the series.  I do understand
> > that this is a bit more hassle right now, but this does help avoid
> > confusion in the future, plus perhaps also avoiding issues with future
> > bisections.
> > 
> We have already decided to get rid of it, i mean small allocations(dynamic
> rcu_head attaching). I will exclude it from next patch-set version. 

Very good, and thank you!!!

							Thanx, Paul