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

[David Hildenbrand]

On 12.11.20 15:37, Vlastimil Babka wrote:
> On 11/11/20 4:38 PM, David Hildenbrand wrote:
> > 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:
>
> Yes, it was, and I thought I fixed it. Guess not.
>
> > return static_branch_unlikely(&_page_poisoning_enabled);
> >
> > > +}
>
> > > @@ -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
>    
> Ok. Fixup below.
>
> ----8<----
>
>   From 7ce26ba61296f583f0f9089e7887f07424f25d2c Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@xxxxxxx>
> Date: Thu, 12 Nov 2020 15:20:58 +0100
> Subject: [PATCH] mm, page_poison: use static key more efficiently-fix
>
> Non-functional cleanups, per David Hildenbrand.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>    include/linux/mm.h | 16 +++++++++++++---
>    mm/page_alloc.c    |  7 +++----
>    mm/page_poison.c   |  4 ++--
>    3 files changed, 18 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 4d6dd9f44571..861b9392b5dc 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2867,8 +2867,8 @@ extern int apply_to_existing_page_range(struct mm_struct *mm,
>    
>    extern void init_mem_debugging_and_hardening(void);
>    #ifdef CONFIG_PAGE_POISONING
> -extern void kernel_poison_pages(struct page *page, int numpages);
> -extern void kernel_unpoison_pages(struct page *page, int numpages);
> +extern void __kernel_poison_pages(struct page *page, int numpages);
> +extern void __kernel_unpoison_pages(struct page *page, int numpages);
>    extern bool _page_poisoning_enabled_early;
>    DECLARE_STATIC_KEY_FALSE(_page_poisoning_enabled);
>    static inline bool page_poisoning_enabled(void)
> @@ -2881,7 +2881,17 @@ static inline bool page_poisoning_enabled(void)
>     */
>    static inline bool page_poisoning_enabled_static(void)
>    {
> -	return (static_branch_unlikely(&_page_poisoning_enabled));
> +	return static_branch_unlikely(&_page_poisoning_enabled);
> +}
> +static inline void kernel_poison_pages(struct page *page, int numpages)
> +{
> +	if (page_poisoning_enabled_static())
> +		__kernel_poison_pages(page, numpages);
> +}
> +static inline void kernel_unpoison_pages(struct page *page, int numpages)
> +{
> +	if (page_poisoning_enabled_static())
> +		__kernel_unpoison_pages(page, numpages);
>    }
>    #else
>    static inline bool page_poisoning_enabled(void) { return false; }
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index fd7f9345adc0..1388b5939551 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1271,8 +1271,8 @@ static __always_inline bool free_pages_prepare(struct page *page,
>    	if (want_init_on_free())
>    		kernel_init_free_pages(page, 1 << order);
>    
> -	if (page_poisoning_enabled_static())
> -		kernel_poison_pages(page, 1 << order);
> +	kernel_poison_pages(page, 1 << order);
> +
>    	/*
>    	 * arch_free_page() can make the page's contents inaccessible.  s390
>    	 * does this.  So nothing which can access the page's contents should
> @@ -2281,8 +2281,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
>    	if (debug_pagealloc_enabled_static())
>    		kernel_map_pages(page, 1 << order, 1);
>    	kasan_alloc_pages(page, order);
> -	if (page_poisoning_enabled_static())
> -		kernel_unpoison_pages(page, 1 << order);
> +	kernel_unpoison_pages(page, 1 << order);
>    	set_page_owner(page, order, gfp_flags);
>    }
>    
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index dd7aeada036f..4d75fc9ccc7a 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -30,7 +30,7 @@ static void poison_page(struct page *page)
>    	kunmap_atomic(addr);
>    }
>    
> -void kernel_poison_pages(struct page *page, int n)
> +void __kernel_poison_pages(struct page *page, int n)
>    {
>    	int i;
>    
> @@ -89,7 +89,7 @@ static void unpoison_page(struct page *page)
>    	kunmap_atomic(addr);
>    }
>    
> -void kernel_unpoison_pages(struct page *page, int n)
> +void __kernel_unpoison_pages(struct page *page, int n)
>    {
>    	int i;
>    

LGTM.

--
Thanks,

David / dhildenb