Re: [PATCH] mm/mmap.c: rb_parent is not necessary in __vma_link_list

[Wei Yang]

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
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