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SubjectRe: [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?

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