Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Paul E. McKenney
2020-08-12T13:31:01Z

On Wed, Aug 12, 2020 at 10:32:50AM +0200, Thomas Gleixner wrote:
> Paul,
>
> "Paul E. McKenney" <paulmck@kernel.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