linux-kernel - Re: [PATCH] mm: Inline vma_needs

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Message-ID: <f7ea4fc4-999a-490d-b221-b3b935e4b1bb@suse.cz>
Date: Wed, 18 Jun 2025 11:39:11 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Yunshui Jiang <jiangyunshui@...inos.cn>, linux-kernel@...r.kernel.org,
 linux-mm@...k.org
Cc: akpm@...ux-foundation.org, david@...hat.com, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com
Subject: Re: [PATCH] mm: Inline vma_needs_copy

On 6/18/25 3:42 AM, Yunshui Jiang wrote:
> From: jiangyunshui <jiangyunshui@...inos.cn>
> 
> Since commit bcd51a3c679d ("hugetlb: lazy page table copies
> in fork()"), the logic about judging whether to copy
> page table inside func copy_page_range has been extracted
> into a separate func vma_needs_copy. While this change
> improves code readability, it also incurs more function call
> overhead, especially where fork() were frequently called.
> 
> Inline func vma_needs_copy to optimize the copy_page_range
> performance. Given that func vma_needs_copy is only called
> by copy_page_range, inlining it would not cause unacceptable
> code bloat.

I'm surprised the compiler doesn't inline it already, if there's a
single caller. In fact, mine (gcc-14.3 on x86) already does.

So I wonder to which extent should we force override wrong compiler
heuristics? Maybe just inline instead of __always_inline would be OK? Is
that enough of a hint for your compiler?

> Testing was done with the byte-unixbench spawn benchmark
> (which frequently calls fork). I measured 1.7% improvement
> on x86 and 1.8% improvement on arm64.
> 
> Signed-off-by: jiangyunshui <jiangyunshui@...inos.cn>
> ---
>  mm/memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 8eba595056fe..d15b07f96ab1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1337,7 +1337,7 @@ copy_p4d_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>   * false when we can speed up fork() by allowing lazy page faults later until
>   * when the child accesses the memory range.
>   */
> -static bool
> +static __always_inline bool
>  vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>  {
>  	/*