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SubjectRe: [PATCH] mm/mmap.c: rb_parent is not necessary in __vma_link_list
On Mon, Aug 12, 2019 at 08:39:58PM -0700, Matthew Wilcox wrote:
>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?
>

I reboot successfully with this patch.

>> @@ -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?

mm->mmap is not the rb_root.

mm->mmap is the first element in the ordered list, if my understanding is
correct.

--
Wei Yang
Help you, Help me

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