Re: [PATCH v5 2/4] mm: Provide kernel parameter to allow disabling page init poisoning

[Alexander Duyck]

On 9/25/2018 3:14 PM, Dave Hansen wrote:
> On 09/25/2018 01:38 PM, Alexander Duyck wrote:
> > On 9/25/2018 1:26 PM, Dave Hansen wrote:
> > > On 09/25/2018 01:20 PM, Alexander Duyck wrote:
> > > > +ÂÂÂ vm_debug[=options]ÂÂÂ [KNL] Available with CONFIG_DEBUG_VM=y.
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ May slow down system boot speed, especially when
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ enabled on systems with a large amount of memory.
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ All options are enabled by default, and this
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ interface is meant to allow for selectively
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ enabling or disabling specific virtual memory
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ debugging features.
> > > > +
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ Available options are:
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂ PÂÂÂ Enable page structure init time poisoning
> > > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂ -ÂÂÂ Disable all of the above options
> > >
> > > Can we have vm_debug=off for turning things off, please? That seems to
> > > be pretty standard.
> >
> > No. The simple reason for that is that you had requested this work like
> > the slub_debug. If we are going to do that then each individual letter
> > represents a feature. That is why the "-" represents off. We cannot have
> > letters represent flags, and letters put together into words. For
> > example slub_debug=OFF would turn on sanity checks and turn off
> > debugging for caches that would have causes higher minimum slab orders.
>
> We don't have to have the same letters mean the same things for both
> options.  We also can live without 'o' and 'f' being valid.  We can
> *also* just say "don't do 'off'" if you want to enable things.

I'm not saying we do either. I would prefer it if we stuck to similar 
behavior though. If we are going to do a slub_debug style parameter then 
we should stick with similar behavior where "-" is used to indicate all 
features off.

> I'd much rather have vm_debug=off do the right thing than have
> per-feature enable/disable.  I know I'll *never* remember vm_debug=- and
> doing it this way will subject me to innumerable trips to Documentation/
> during my few remaining years.
>
> Surely you can make vm_debug=off happen. :)

I could, but then it is going to confuse people even more. I really feel 
that if we want to do a slub_debug style interface we should use the 
same switch for turning off all the features that they do for slub_debug.

> > > we need to document the defaults. I think the default is "all
> > > debug options are enabled", but it would be nice to document that.
> >
> > In the description I call out "All options are enabled by default, and this interface is meant to allow for selectively enabling or disabling".
>
> I found "all options are enabled by default" really confusing.  Maybe:
>
> "Control debug features which become available when CONFIG_DEBUG_VM=y.
> When this option is not specified, all debug features are enabled.  Use
> this option enable a specific subset."
>
> Then, let's actually say what the options do, and what their impact is:
>
> 	P	Enable 'struct page' poisoning at initialization.
> 		(Slows down boot time).

From my perspective I just don't see how this changes much since it 
conveys the same message I had conveyed in my description. Since it 
looks like Andrew applied the patch feel free to submit your suggestion 
here as a follow-up patch and I would be willing to review/ack it.