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

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Date:   Thu, 13 Aug 2020 09:29:04 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

On Thu, Aug 13, 2020 at 06:13:57PM +0200, Michal Hocko wrote:
> On Thu 13-08-20 09:04:42, Paul E. McKenney wrote:
> > On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko wrote:
> [...]
> > > If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity
> > > check then there is no functional problem - GFP_RT_SAFE would still be
> > > GFP_NOWAIT so functional wise the allocator will still do the right
> > > thing.
> > 
> > Perhaps it was just me getting confused, early hour Pacific Time and
> > whatever other excuses might apply.  But I thought that you still had
> > an objection to GFP_RT_SAFE based on changes in allocator semantics for
> > other users.
> 
> There is still that problem with lockdep complaining about raw->regular
> spinlock on !PREEMPT_RT that would need to get resolved somehow. Thomas
> is not really keen on adding some lockdep annotation mechanism and
> unfortunatelly I do not have a different idea how to get rid of those.

OK.  So the current situation requires a choice between these these
alternatives, each of which has shortcomings that have been mentioned
earlier in this thread:

1.	Prohibit invoking allocators from raw atomic context, such
	as when holding a raw spinlock.

2.	Adding a GFP_ flag.

3.	Reusing existing GFP_ flags/values/whatever to communicate
	the raw-context information that was to be communicated by
	the new GFP_ flag.

4.	Making lockdep forgive acquiring spinlocks while holding
	raw spinlocks, but only in CONFIG_PREEMPT_NONE=y kernels.

Am I missing anything?

						Thanx, Paul