linux-kernel - Re: [PATCH/RFC 3/3] s390: query dynamic DEBUG

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Date:	Tue, 26 Jan 2016 15:29:38 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-s390@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH/RFC 3/3] s390: query dynamic DEBUG_PAGEALLOC setting

On Tue, 26 Jan 2016, Heiko Carstens wrote:

> On Tue, Jan 26, 2016 at 10:18:25AM +0100, Christian Borntraeger wrote:
> > We can use debug_pagealloc_enabled() to check if we can map
> > the identity mapping with 1MB/2GB pages as well as to print
> > the current setting in dump_stack.
> > 
> > Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
> > ---
> >  arch/s390/kernel/dumpstack.c |  4 +++-
> >  arch/s390/mm/vmem.c          | 10 ++++------
> >  2 files changed, 7 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/s390/kernel/dumpstack.c b/arch/s390/kernel/dumpstack.c
> > index dc8e204..a1c0530 100644
> > --- a/arch/s390/kernel/dumpstack.c
> > +++ b/arch/s390/kernel/dumpstack.c
> > @@ -11,6 +11,7 @@
> >  #include <linux/export.h>
> >  #include <linux/kdebug.h>
> >  #include <linux/ptrace.h>
> > +#include <linux/mm.h>
> >  #include <linux/module.h>
> >  #include <linux/sched.h>
> >  #include <asm/processor.h>
> > @@ -186,7 +187,8 @@ void die(struct pt_regs *regs, const char *str)
> >  	printk("SMP ");
> >  #endif
> >  #ifdef CONFIG_DEBUG_PAGEALLOC
> > -	printk("DEBUG_PAGEALLOC");
> > +	printk("DEBUG_PAGEALLOC(%s)",
> > +		debug_pagealloc_enabled() ? "enabled" : "disabled");
> >  #endif
> 
> I'd prefer if you change this to
> 
> 	if (debug_pagealloc_enabled())
> 		printk("DEBUG_PAGEALLOC");
> 
> That way we can get rid of yet another ifdef. Having
> "DEBUG_PAGEALLOC(disabled)" doesn't seem to be very helpful.
> 

I'd agree if CONFIG_DEBUG_PAGEALLOC only did anything when 
debug_pagealloc_enabled() is true, but that doesn't seem to be the case.  
When CONFIG_DEBUG_SLAB is enabled, for instance, CONFIG_DEBUG_PAGEALLOC 
also enables stackinfo storing and poisoning and it's not guarded by 
debug_pagealloc_enabled().

It seems like CONFIG_DEBUG_PAGEALLOC enables debugging functionality 
outside the scope of the debug_pagealloc=on kernel parameter, so 
DEBUG_PAGEALLOC(disabled) actually does mean something.