linux-kernel - Re: [PATCH 2/3] mm, page_poison: use static key more efficiently

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Date:   Tue, 27 Oct 2020 10:07:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...nel.org>,
        Mateusz Nosek <mateusznosek0@...il.com>
Subject: Re: [PATCH 2/3] mm, page_poison: use static key more efficiently

On 26.10.20 18:33, Vlastimil Babka wrote:
> Commit 11c9c7edae06 ("mm/page_poison.c: replace bool variable with static key")
> changed page_poisoning_enabled() to a static key check. However, the function
> is not inlined, so each check still involves a function call with overhead not
> eliminated when page poisoning is disabled.
> 
> Analogically to how debug_pagealloc is handled, this patch converts
> page_poisoning_enabled() back to boolean check, and introduces
> page_poisoning_enabled_static() for fast paths. Both functions are inlined.
> 
> Also optimize the check that enables page poisoning instead of debug_pagealloc
> for architectures without proper debug_pagealloc support. Move the check to
> init_mem_debugging() to enable a single static key instead of having two
> static branches in page_poisoning_enabled_static().
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>

[...]

> +/*
> + * For use in fast paths after init_mem_debugging() has run, or when a
> + * false negative result is not harmful when called too early.
> + */
> +static inline bool page_poisoning_enabled_static(void)
> +{
> +	return (static_branch_unlikely(&_page_poisoning_enabled));


return static_branch_unlikely(&_page_poisoning_enabled);

> +}
>   #else
>   static inline bool page_poisoning_enabled(void) { return false; }
> +static inline bool page_poisoning_enabled_static(void) { return false; }
>   static inline void kernel_poison_pages(struct page *page, int numpages,
>   					int enable) { }
>   #endif
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b168c58ef337..2a1be197649d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -777,6 +777,17 @@ void init_mem_debugging()
>   		}
>   	}
>   
> +#ifdef CONFIG_PAGE_POISONING
> +	/*
> +	 * Page poisoning is debug page alloc for some arches. If
> +	 * either of those options are enabled, enable poisoning.
> +	 */
> +	if (page_poisoning_enabled() ||
> +			(!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
> +			debug_pagealloc_enabled()))

Weird indentation

if (page_poisoning_enabled() ||
     (!IS_ENABLED(CONFIG_ARCH_SUPPORTS_DEBUG_PAGEALLOC) &&
      debug_pagealloc_enabled()))


> +		static_branch_enable(&_page_poisoning_enabled);
> +#endif
> +
>   #ifdef CONFIG_DEBUG_PAGEALLOC
>   	if (!debug_pagealloc_enabled())
>   		return;
> @@ -2208,7 +2219,7 @@ static inline int check_new_page(struct page *page)
>   static inline bool free_pages_prezeroed(void)
>   {
>   	return (IS_ENABLED(CONFIG_PAGE_POISONING_ZERO) &&
> -		page_poisoning_enabled()) || want_init_on_free();
> +		page_poisoning_enabled_static()) || want_init_on_free();
>   }

Apart from that LGTM.

-- 
Thanks,

David / dhildenb