Re: [PATCH] mm/debug_pagealloc: Ask users for default setting of debug_pagealloc

[Heiko Carstens]

On Mon, Jan 25, 2016 at 10:45:50AM +0100, Christian Borntraeger wrote:
> >> +	  By default this option will be almost for free and can be activated
> >> +	  in distribution kernels. The overhead and the debugging can be enabled
> >> +	  by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc command line
> >> +	  parameter.
> > 
> > Sorry, but it's not almost for free and should not be used by distribution
> > kernels. If we have DEBUG_PAGEALLOC enabled, at least on s390 we will not
> > make use of 2GB and 1MB pagetable entries for the identy mapping anymore.
> > Instead we will only use 4K mappings.
> 
> Hmmm, can we change these code areas to use debug_pagealloc_enabled? I guess
> this evaluated too late?

Yes, that should be possible. "debug_pagealloc" is an early_param, which
will be evaluated before we call paging_init() (both in
arch/s390/kernel/setup.c).

So it looks like this can be trivially changed. (replace the ifdefs in
arch/s390/mm/vmem.c with debug_pagealloc_enabled()).

> > I assume this is true for all architectures since freeing pages can happen
> > in any context and therefore we can't allocate memory in order to split
> > page tables.
> > 
> > So enabling this will cost memory and put more pressure on the TLB.
> 
> So I will change the description and drop the "if unsure" statement.

Well, given that we can change it like above... I don't care anymore ;)