Re: [PATCH] mm/mmap.c: refine data locality of find_vma_prev

From: Balbir Singh <sblbir@amzn.com>
To: Wei Yang <richardw.yang@linux.intel.com>
Cc: <akpm@linux-foundation.org>, <mhocko@suse.com>, <vbabka@suse.cz>,
	<kirill.shutemov@linux.intel.com>, <linux-mm@kvack.org>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm/mmap.c: refine data locality of find_vma_prev
Date: Tue, 6 Aug 2019 10:58:22 +0000	[thread overview]
Message-ID: <20190806105822.GA25354@dev-dsk-sblbir-2a-88e651b2.us-west-2.amazon.com> (raw)
In-Reply-To: <20190806081123.22334-1-richardw.yang@linux.intel.com>

On Tue, Aug 06, 2019 at 04:11:23PM +0800, Wei Yang wrote:
> When addr is out of the range of the whole rb_tree, pprev will points to
> the biggest node. find_vma_prev gets is by going through the right most
> node of the tree.
> 
> Since only the last node is the one it is looking for, it is not
> necessary to assign pprev to those middle stage nodes. By assigning
> pprev to the last node directly, it tries to improve the function
> locality a little.
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
>  mm/mmap.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 7e8c3e8ae75f..284bc7e51f9c 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2271,11 +2271,10 @@ find_vma_prev(struct mm_struct *mm, unsigned long addr,
>  		*pprev = vma->vm_prev;
>  	} else {
>  		struct rb_node *rb_node = mm->mm_rb.rb_node;
> -		*pprev = NULL;
> -		while (rb_node) {
> -			*pprev = rb_entry(rb_node, struct vm_area_struct, vm_rb);
> +		while (rb_node && rb_node->rb_right)
>  			rb_node = rb_node->rb_right;
> -		}
> +		*pprev = rb_node ? NULL
> +			 : rb_entry(rb_node, struct vm_area_struct, vm_rb);

Can rb_node ever be NULL? assuming mm->mm_rb.rb_node is not NULL when we
enter here

Balbir Singh