Re: [PATCH 6/9] mm, slub: introduce static key for slub_debug()

[Kees Cook <keescook@chromium.org>]

On Wed, Jun 10, 2020 at 06:31:32PM +0200, Vlastimil Babka wrote:
> One advantage of CONFIG_SLUB_DEBUG is that a generic distro kernel can be built
> with the option enabled, but it's inactive until simply enabled on boot,
> without rebuilding the kernel. With a static key, we can further eliminate the
> overhead of checking whether a cache has a particular debug flag enabled if we
> know that there are no such caches (slub_debug was not enabled during boot). We
> use the same mechanism also for e.g.  page_owner, debug_pagealloc or kmemcg
> functionality.
> 
> This patch introduces the static key and makes the general check for per-cache
> debug flags kmem_cache_debug() use it. This benefits several call sites,
> including (slow path but still rather frequent) __slab_free(). The next patches
> will add more uses.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 8dd2925448ae..24d3e5f832aa 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -114,13 +114,21 @@
>   * 			the fast path and disables lockless freelists.
>   */
>  
> +#ifdef CONFIG_SLUB_DEBUG
> +#ifdef CONFIG_SLUB_DEBUG_ON
> +DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
> +#else
> +DEFINE_STATIC_KEY_FALSE(slub_debug_enabled);
> +#endif
> +#endif

Yet another potential future user of DEFINE_STATIC_KEY_MAYBE()[1], once
I get it landed. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

[1] https://lore.kernel.org/lkml/20200324203231.64324-2-keescook@chromium.org/

-- 
Kees Cook