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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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:

s/hods/holds/

>    vma->vm_start < start + len = end
> 
> This means the if check would never be true.

Have you considered overflow?

> This patch removes this redundant check and fix two typo in comment.
> 
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> ---
>  mm/mmap.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 8d6449e74431..94660ddfa2c1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -414,7 +414,7 @@ static void vma_gap_update(struct vm_area_struct *vma)
>  {
>  	/*
>  	 * As it turns out, RB_DECLARE_CALLBACKS() already created a callback
> -	 * function that does exacltly what we want.
> +	 * function that does exactly what we want.
>  	 */
>  	vma_gap_callbacks_propagate(&vma->vm_rb, NULL);
>  }
> @@ -1621,7 +1621,7 @@ SYSCALL_DEFINE1(old_mmap, struct mmap_arg_struct __user *, arg)
>  #endif /* __ARCH_WANT_SYS_OLD_MMAP */
>  
>  /*
> - * Some shared mappigns will want the pages marked read-only
> + * Some shared mappings will want the pages marked read-only
>   * to track write events. If so, we'll downgrade vm_page_prot
>   * to the private version (using protection_map[] without the
>   * VM_SHARED bit).
> @@ -2705,12 +2705,8 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len,
>  	if (!vma)
>  		return 0;
>  	prev = vma->vm_prev;
> -	/* 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;
>  
>  	/*
>  	 * If we need to split any vma, do it now to save pain later.
> -- 
> 2.15.1
> 

-- 
 Kirill A. Shutemov