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

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Message-ID: <20200812133045.GE4295@paulmck-ThinkPad-P72>
Date:   Wed, 12 Aug 2020 06:30:45 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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 Wed, Aug 12, 2020 at 10:32:50AM +0200, Thomas Gleixner wrote:
> Paul,
> 
> "Paul E. McKenney" <paulmck@...nel.org> writes:
> > On Wed, Aug 12, 2020 at 02:13:25AM +0200, Thomas Gleixner wrote:
> >> That much I understood, but I somehow failed to figure the why out
> >> despite the elaborate changelog. 2 weeks of 30+C seem to have cooked my
> >> brain :)
> >
> > Ouch!!!  And what on earth is Germany doing being that warm???
> 
> The hot air exhaustion of politicians, managers and conspiracy
> mythomaniacs seens to have contributed extensivly to global warming
> lately.

In that case, our only hope here in this geography is that we are in a
simulation, so that the hot air will cause a signed integer overflow to
negative numbers some fraction of the time.  :-(

> >> But what makes me really unhappy is that my defense line against
> >> allocations from truly atomic contexts (from RT POV) which was enforced
> >> on RT gets a real big gap shot into it.
> >
> > Understood, and agreed:  We do need to keep the RT degradation in
> > check.
> 
> Not only that. It's bad practice in general to do memory allocations
> from such contexts if not absolutely necessary and the majority of cases
> which we cleaned up over time were just from the "works for me and why
> should I care and start to think" departement.

Agreed, and I continue to see some of that myself.  :-/

> >> I can understand your rationale and what you are trying to solve. So, if
> >> we can actually have a distinct GFP variant:
> >> 
> >>   GFP_I_ABSOLUTELY_HAVE_TO_DO_THAT_AND_I_KNOW_IT_CAN_FAIL_EARLY
> >> 
> >> which is easy to grep for then having the page allocator go down to the
> >> point where zone lock gets involved is not the end of the world for
> >> RT in theory - unless that damned reality tells otherwise. :)
> >
> > I have no objection to an otherwise objectionable name in this particular
> > case.  After all, we now have 100 characters per line, right?  ;-)
> 
> Hehe. I can live with the proposed NO_LOCK name or anything distinct
> which the mm people can agree on.

Sounds good.  ;-)

> >> To make it consistent the same GFP_ variant should allow the slab
> >> allocator go to the point where the slab cache is exhausted.
> >
> > Why not wait until someone has an extremely good reason for needing
> > this functionality from the slab allocators?  After all, leaving out
> > the slab allocators would provide a more robust defense line.  Yes,
> > consistent APIs are very good things as a general rule, but maybe this
> > situation is one of the exceptions to that rule.
> 
> Fair enough.
> 
> >> Having a distinct and clearly defined GFP_ variant is really key to
> >> chase down offenders and to make reviewers double check upfront why this
> >> is absolutely required.
> >
> > Checks for that GFP_ variant could be added to automation, though reality
> > might eventually prove that to be a mixed blessing.
> 
> Did you really have to remind me and destroy my illusions before I was
> able to marvel at them?

Apologies!  I am afraid that it has become a reflex due to living in
this time and place.  My further fear is that I will have all to great
an opportunity for further reinforcing this reflex in the future.  :-/

							Thanx, Paul