linux-kernel - Re: [PATCH v4 1/5] mm: Move pfn_to_online

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Date:   Wed, 20 Jan 2021 11:11:12 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/5] mm: Move pfn_to_online_page() out of line

On Wed 13-01-21 16:43:16, Dan Williams wrote:
> pfn_to_online_page() is already too large to be a macro or an inline
> function. In anticipation of further logic changes / growth, move it out
> of line.
> 
> No functional change, just code movement.
> 
> Reported-by: Michal Hocko <mhocko@...nel.org>

I am not sure what r-b refers to. I suspect it is the overal problem
rather than this particular patch. Not that I care much but it might get
confusing because I do not remember ever complaining about
pfn_to_online_page to be too large.

> Reviewed-by: David Hildenbrand <david@...hat.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>

I do not remember any hot path which depends on pfn walk and a function
call would be clearly visible. If this ever turn out  to be a problem we
can make it inline and push the heavy lifting out of line.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

> ---
>  include/linux/memory_hotplug.h |   17 +----------------
>  mm/memory_hotplug.c            |   16 ++++++++++++++++
>  2 files changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
> index 15acce5ab106..3d99de0db2dd 100644
> --- a/include/linux/memory_hotplug.h
> +++ b/include/linux/memory_hotplug.h
> @@ -16,22 +16,7 @@ struct resource;
>  struct vmem_altmap;
>  
>  #ifdef CONFIG_MEMORY_HOTPLUG
> -/*
> - * Return page for the valid pfn only if the page is online. All pfn
> - * walkers which rely on the fully initialized page->flags and others
> - * should use this rather than pfn_valid && pfn_to_page
> - */
> -#define pfn_to_online_page(pfn)					   \
> -({								   \
> -	struct page *___page = NULL;				   \
> -	unsigned long ___pfn = pfn;				   \
> -	unsigned long ___nr = pfn_to_section_nr(___pfn);	   \
> -								   \
> -	if (___nr < NR_MEM_SECTIONS && online_section_nr(___nr) && \
> -	    pfn_valid_within(___pfn))				   \
> -		___page = pfn_to_page(___pfn);			   \
> -	___page;						   \
> -})
> +struct page *pfn_to_online_page(unsigned long pfn);
>  
>  /*
>   * Types for free bootmem stored in page->lru.next. These have to be in
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index f9d57b9be8c7..55a69d4396e7 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -300,6 +300,22 @@ static int check_hotplug_memory_addressable(unsigned long pfn,
>  	return 0;
>  }
>  
> +/*
> + * Return page for the valid pfn only if the page is online. All pfn
> + * walkers which rely on the fully initialized page->flags and others
> + * should use this rather than pfn_valid && pfn_to_page
> + */
> +struct page *pfn_to_online_page(unsigned long pfn)
> +{
> +	unsigned long nr = pfn_to_section_nr(pfn);
> +
> +	if (nr < NR_MEM_SECTIONS && online_section_nr(nr) &&
> +	    pfn_valid_within(pfn))
> +		return pfn_to_page(pfn);
> +	return NULL;
> +}
> +EXPORT_SYMBOL_GPL(pfn_to_online_page);
> +
>  /*
>   * Reasonably generic function for adding memory.  It is
>   * expected that archs that support memory hotplug will

-- 
Michal Hocko
SUSE Labs