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SubjectRe: [PATCH v2 2/5] mm, page_poison: use static key more efficiently
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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

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