Re: [PATCH] mm/slub: fix help comment of SLUB_DEBUG[_ON]

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, Feb 23, 2023 at 07:20:58AM +0800, Vernon Yang wrote:
> Since commit 081248de0a02 ("kset: move /sys/slab to /sys/kernel/slab"),
> the SYSFS path of slab was modified, so fix up the help comment of
> SLUB_DEBUG to use the correct one.
> 
> And now the "slub_debug" parameter on boot that is support for more
> fine grained debug control, so fix up help comment of SLUB_DEBUG_ON
> for "no support" to "support".
> 
> Signed-off-by: Vernon Yang <vernon2gm@gmail.com>
> ---
>  mm/Kconfig.debug | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/Kconfig.debug b/mm/Kconfig.debug
> index fca699ad1fb0..732043bf56c6 100644
> --- a/mm/Kconfig.debug
> +++ b/mm/Kconfig.debug
> @@ -61,8 +61,8 @@ config SLUB_DEBUG
>  	help
>  	  SLUB has extensive debug support features. Disabling these can
>  	  result in significant savings in code size. This also disables
> -	  SLUB sysfs support. /sys/slab will not exist and there will be
> -	  no support for cache validation etc.
> +	  SLUB sysfs support. /sys/kernel/slab will not exist and there
> +	  will be no support for cache validation etc.

After commit ab4d5ed5eeda ("slub: Enable sysfs support for !CONFIG_SLUB_DEBUG"),
"Disabling SLUB_DEBUG also disables SLUB sysfs support" is not true anymore.

>  config SLUB_DEBUG_ON
>  	bool "SLUB debugging on by default"
> @@ -73,10 +73,10 @@ config SLUB_DEBUG_ON
>  	  Boot with debugging on by default. SLUB boots by default with
>  	  the runtime debug capabilities switched off. Enabling this is
>  	  equivalent to specifying the "slub_debug" parameter on boot.
> -	  There is no support for more fine grained debug control like
> -	  possible with slub_debug=xxx. SLUB debugging may be switched
> -	  off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying
> -	  "slub_debug=-".
> +	  There is support for more fine grained debug control like
> +	  possible with slub_debug=xxx, details in Documentation/mm/slub.rst.
> +	  SLUB debugging may be switched off in a kernel built with
> +	  CONFIG_SLUB_DEBUG_ON by specifying "slub_debug=-".

I think this is a misunderstanding. SLUB_DEBUG_ON is equivalent to
passing boot parameter slub_debug=FUPZ, and what the help text says is that

"You can enable slub debugging by default using this config option but
 there is no support for fine-grained control like what slub_debug
 boot parameter provides."

 IOW SLUB_DEBUG_ON cannot be used to have same effect as passing
 something like slub_debug=F.

>  
>  config PAGE_OWNER
>  	bool "Track page owner"
> -- 
> 2.34.1
> 
>