linux-kernel - Re: [PATCH v2 2/5] mm, page_poison: use static key more efficiently

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Date:   Wed, 11 Nov 2020 16:38:31 +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>,
        Laura Abbott <labbott@...nel.org>
Subject: Re: [PATCH v2 2/5] mm, page_poison: use static key more efficiently

On 03.11.20 16:22, 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.
> 
> The function kernel_poison_pages() is also called unconditionally and does
> the static key check inside. Remove it from there and put it to callers. Also
> split it to two functions kernel_poison_pages() and kernel_unpoison_pages()
> instead of the confusing bool parameter.
> 
> 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_and_hardening() to enable a single static key instead of
> having two static branches in page_poisoning_enabled_static().

[...]

> + * 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));

As already mentioned IIRC:

return static_branch_unlikely(&_page_poisoning_enabled);

> +}
>   #else
>   static inline bool page_poisoning_enabled(void) { return false; }
> -static inline void kernel_poison_pages(struct page *page, int numpages,
> -					int enable) { }
> +static inline bool page_poisoning_enabled_static(void) { return false; }
> +static inline void kernel_poison_pages(struct page *page, int numpages) { }
> +static inline void kernel_unpoison_pages(struct page *page, int numpages) { }
>   #endif
>   
>   DECLARE_STATIC_KEY_FALSE(init_on_alloc);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 44d596c9c764..fd7f9345adc0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -775,6 +775,17 @@ void init_mem_debugging_and_hardening(void)
>   			static_branch_enable(&init_on_free);
>   	}
>   
> +#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()))
> +		static_branch_enable(&_page_poisoning_enabled);
> +#endif
> +
>   #ifdef CONFIG_DEBUG_PAGEALLOC
>   	if (!debug_pagealloc_enabled())
>   		return;
> @@ -1260,7 +1271,8 @@ static __always_inline bool free_pages_prepare(struct page *page,
>   	if (want_init_on_free())
>   		kernel_init_free_pages(page, 1 << order);
>   
> -	kernel_poison_pages(page, 1 << order, 0);
> +	if (page_poisoning_enabled_static())
> +		kernel_poison_pages(page, 1 << order);

This would look much better by having kernel_poison_pages() simply be 
implemented in a header, where the static check is performed.

Take a look at how it's handled in mm/shuffle.h

-- 
Thanks,

David / dhildenb