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

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Message-ID: <20160127005920.GB7089@js1304-P5Q-DELUXE>
Date:	Wed, 27 Jan 2016 09:59:21 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	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, Jan 26, 2016 at 04:36:11PM -0800, David Rientjes wrote:
> On Wed, 27 Jan 2016, Joonsoo Kim wrote:
> 
> > > 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.
> > 
> > Hello, David.
> > 
> > I tried to fix CONFIG_DEBUG_SLAB case on 04/16 of following patchset.
> > 
> > http://thread.gmane.org/gmane.linux.kernel.mm/144527
> > 
> > I found that there are more sites to fix but not so many.
> > We can do it.
> > 
> 
> For the slab case, sure, this can be fixed, but there is other code that 
> uses CONFIG_DEBUG_PAGEALLOC to suggest debugging is always enabled and is 
> indifferent to debug_pagealloc_enabled().  I find this in powerpc and 
> sparc arch code as well as generic vmalloc code.  

Yes, I also found it.

> 
> If we can convert existing users that only check for 
> CONFIG_DEBUG_PAGEALLOC to rather check for debug_pagealloc_enabled() and 
> agree that it is only enabled for debug_pagealloc=on, then this would seem 
> fine.  However, I think we should at least consult with those users before 
> removing an artifact from the kernel log that could be useful in debugging 
> why a particular BUG() happened.

Yes, at least, non-architecture dependent code (vmalloc, SLAB, SLUB) should
be changed first. If Christian doesn't mind, I will try to fix above 3
things.

Thanks.