linux-kernel - Re: [PATCH] mm/mmap.c: Remove redundent 'get_area' function pointer in get_unmapped

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Date:	Thu, 10 Sep 2015 15:32:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	gang.chen.5i5j@...il.com
Cc:	mhocko@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	gchen_5i5j@...n.com
Subject: Re: [PATCH] mm/mmap.c: Remove redundent 'get_area' function pointer
 in get_unmapped_area()

On Thu,  3 Sep 2015 12:14:51 +0800 gang.chen.5i5j@...il.com wrote:

> From: Chen Gang <gang.chen.5i5j@...il.com>
> 
> Call the function pointer directly, then let code a bit simpler.
> 
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2012,10 +2012,8 @@ unsigned long
>  get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  		unsigned long pgoff, unsigned long flags)
>  {
> -	unsigned long (*get_area)(struct file *, unsigned long,
> -				  unsigned long, unsigned long, unsigned long);
> -
>  	unsigned long error = arch_mmap_check(addr, len, flags);
> +
>  	if (error)
>  		return error;
>  
> @@ -2023,10 +2021,12 @@ get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
>  	if (len > TASK_SIZE)
>  		return -ENOMEM;
>  
> -	get_area = current->mm->get_unmapped_area;
>  	if (file && file->f_op->get_unmapped_area)
> -		get_area = file->f_op->get_unmapped_area;
> -	addr = get_area(file, addr, len, pgoff, flags);
> +		addr = file->f_op->get_unmapped_area(file, addr, len,
> +							pgoff, flags);
> +	else
> +		addr = current->mm->get_unmapped_area(file, addr, len,
> +							pgoff, flags);
>  	if (IS_ERR_VALUE(addr))
>  		return addr;

size(1) says this generates more object code.  And that probably means
slightly worse code.  I didn't investigate, but probably the compiler
is now preparing those five args at two different sites.

Which is pretty dumb of it - the compiler could have stacked the args
first, then chosen the appropriate function to call.

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