linux-kernel - Re: [PATCH] mm/mmap.c: rb_parent is not necessary in __vma_link

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Date:   Mon, 12 Aug 2019 20:39:58 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Yang <richardw.yang@...ux.intel.com>
Cc:     akpm@...ux-foundation.org, mgorman@...hsingularity.net,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmap.c: rb_parent is not necessary in __vma_link_list

On Tue, Aug 13, 2019 at 11:26:56AM +0800, Wei Yang wrote:
> Now we use rb_parent to get next, while this is not necessary.
> 
> When prev is NULL, this means vma should be the first element in the
> list. Then next should be current first one (mm->mmap), no matter
> whether we have parent or not.
> 
> After removing it, the code shows the beauty of symmetry.

Uhh ... did you test this?

> @@ -273,12 +273,8 @@ void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma,
>  		next = prev->vm_next;
>  		prev->vm_next = vma;
>  	} else {
> +		next = mm->mmap;
>  		mm->mmap = vma;
> -		if (rb_parent)
> -			next = rb_entry(rb_parent,
> -					struct vm_area_struct, vm_rb);
> -		else
> -			next = NULL;
>  	}

The full context is:

        if (prev) {
                next = prev->vm_next;
                prev->vm_next = vma;
        } else {
                mm->mmap = vma;
                if (rb_parent)
                        next = rb_entry(rb_parent,
                                        struct vm_area_struct, vm_rb);
                else
                        next = NULL;
        }

Let's imagine we have a small tree with three ranges in it.

A: 5-7
B: 8-10
C: 11-13

I would imagine an rbtree for this case has B at the top with A
to its left and B to its right.

Now we're going to add range D at 3-4.  'next' should clearly be range A.
It will have NULL prev.  Your code is going to make 'B' next, not A.
Right?