linux-kernel - Re: [RFC-PATCH 1/2] mm: Add __GFP_NO

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Date:   Sat, 15 Aug 2020 07:23:25 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Michal Hocko <mhocko@...e.com>,
        Uladzislau Rezki <urezki@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Sat, Aug 15, 2020 at 07:18:39AM -0700, Paul E. McKenney wrote:
> On Sat, Aug 15, 2020 at 10:42:50AM +0200, Peter Zijlstra wrote:
> > On Sat, Aug 15, 2020 at 01:14:53AM +0200, Thomas Gleixner wrote:
> > 
> > > #1 trivial fix is to force switching to an high prio thread or a soft
> > >    interrupt which does the allocation
> > 
> > Yeah, push the alocation out to another context. I did consider it, but
> > why bother?
> > 
> > Also, raising a softirq can't be done from every context, that's a whole
> > new problem. You can do irq_work I suppose, but not all architectures
> > support the self-IPI yet.
> > 
> > All in all, it's just more complexity than the fairly trivial
> > __alloc_page_lockless().
> > 
> > Whichever way around, we can't rely on the allocation.
> 
> One way to enforce that would be to put something like this at the
> beginning of the __alloc_page_lockless() function:
> 
> 	if (IS_ENABLED(CONFIG_PROVE_LOCKING) && (prandom_u32() & 0xffff))
> 		return NULL;

Right, too early in the morning.  :-/

This "slight" variation might include a bit of usefulness along with
the convincing:

	if (IS_ENABLED(CONFIG_PROVE_LOCKING) && !(prandom_u32() & 0xff))
		return NULL;

Plus failing one out of 256 times is likely a better choice than once
out of 65536 times, especially for the occasional caller of this
function.

							Thanx, Paul

> I am sure that there is a better choice than CONFIG_PROVE_LOCKING.
> But whatever the choice, there is nothing quite like the occasional
> allocation failure during testing to convince people that such failure
> really can happen.
> 
> 							Thanx, Paul