Re: [PATCH] mm: remove a redundant check in do_munmap()

[Matthew Wilcox <willy@infradead.org>]

On Wed, Oct 10, 2018 at 08:53:27PM +0800, Wei Yang wrote:
> A non-NULL vma returned from find_vma() implies:
> 
>    vma->vm_start <= start
> 
> Since len != 0, the following condition always hods:
> 
>    vma->vm_start < start + len = end
> 
> This means the if check would never be true.

This is true because earlier in the function, start + len is checked to
be sure that it does not wrap.

> This patch removes this redundant check and fix two typo in comment.

> @@ -2705,12 +2705,8 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
> -	/* we have  start < vma->vm_end  */
> -
> -	/* if it doesn't overlap, we have nothing.. */
> +	/* we have vma->vm_start <= start < vma->vm_end */
>  	end = start + len;
> -	if (vma->vm_start >= end)
> -		return 0;

I agree that it's not currently a useful check, but it's also not going
to have much effect on anything to delete it.  I think there are probably
more worthwhile places to look for inefficiencies.