linux-kernel - Re: [PATCH/RFC] mm/debug_pagealloc: enable it by default

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Message-ID: <20160125064151.GA22557@js1304-P5Q-DELUXE>
Date:	Mon, 25 Jan 2016 15:41:51 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	heiko.carstens@...ibm.com, akpm@...ux-foundation.org
Subject: Re: [PATCH/RFC] mm/debug_pagealloc: enable it by default

Hello,

On Fri, Jan 22, 2016 at 12:44:40PM +0100, Christian Borntraeger wrote:
> since commit 031bc5743f158 ("mm/debug-pagealloc: make debug-pagealloc
> boottime configurable") CONFIG_DEBUG_PAGEALLOC is by default a no-op.
> 
> This resulted in several unnoticed bugs, e.g.
> https://lkml.kernel.org/g/<569F5E29.3090107@...ibm.com>
> or
> https://lkml.kernel.org/g/<56A20F30.4050705@...ibm.com>
> 
> as this behaviour change was not even documented in Kconfig.
> 
> Let's go back to the original default and reverse the command
> line parameter to allow disabling the feature.

Default disable is requested by Andrew and I agreed that.

https://lkml.org/lkml/2014/11/27/3

I think that default disable is good thing because we can use
the kernel compiled with CONFIG_DEBUG_PAGEALLOC even for production.
Unless boot param is provided, it's runtime cost is nearly zero.
This could help to debug in some situations and make this debugging
feature more useful. But, yes, old users can be surprised
by this change. How about changing Kconfig to ask user for default mode?

Thanks.