Re: [PATCH] mm/vmscan: make do_shrink_slab more robust.

[<jiang.biao2@zte.com.cn>]

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On Mon, Nov 27, 2017 at 02:27:20PM +0800, jiang.biao2@zte.com.cn wrote:> > I agree with your concern.  How about we take another way by
> > adding some warning in such case? such as,
> >         freeable = shrinker->count_objects(shrinker, shrinkctl);
> > +       if (unlikely(freeable < 0)) {
> > +               pr_err("shrink_slab: %pF negative objects returned. freeable=%ld\n",
> > +                       shrinker->scan_objects, freeable);
> > +               freeable = 0;  //maybe not needed?
> > +       }
> >         if (freeable == 0)
> >                 return 0;
> > In this way, we would not break the API, but could alert user exception
> > with message, and make it more robust in such case.
>
> True but it would be a problem robust vs. effectivess tradeoff.
> Think about that everyone want to make thier code robust.
> It means they start to dump lots of defensive code so code start
> to look like complicated as well as binary bloating.
> So, whenever we add some more, we should think how effective
> the code I am putting?
> 
> In this case, I'm skeptical, Sorry. But others might have different
> opinions. :)

With all due respect. I still think the robustness is more important than 
effectiveness in this case. :)
First, the minus value could break all the following procedure in 
do_shrink_slab which may lead to long time looping on 
*tight on memory* situations.
Second, there is *unlikely* macro to minimize the cost.
Third, there is already similar procedure for *total_scan < 0*.
Last, it did happen in my scenario and maybe happen in other 
unexpected situations.

Just my personal option, thanks a lot for you reply and explanation. :)

Regards,