linux-kernel - Re: [PATCH] mm: mmap: remove unnecessary unlikely()

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Date:   Mon, 5 Nov 2018 07:51:05 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yangtao Li <tiny.windzz@...il.com>
Cc:     akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        vbabka@...e.cz, yang.shi@...ux.alibaba.com, rientjes@...gle.com,
        linux@...inikbrodowski.net, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: mmap: remove unnecessary unlikely()

On Sun 04-11-18 07:44:56, Yangtao Li wrote:
> WARN_ON() already contains an unlikely(), so it's not necessary to use
> unlikely.

We should just get rid of this ugliness altogether. It no longer serves
its purpose.  This is a historical artifact from 2005 where do_brk
was called outside of the core mm. We do have a proper abstraction in
vm_brk_flags and that one does the locking properly.

> 
> Signed-off-by: Yangtao Li <tiny.windzz@...il.com>
> ---
>  mm/mmap.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 6c04292e16a7..2077008ade0c 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2965,10 +2965,8 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
>  static inline void verify_mm_writelocked(struct mm_struct *mm)
>  {
>  #ifdef CONFIG_DEBUG_VM
> -	if (unlikely(down_read_trylock(&mm->mmap_sem))) {
> -		WARN_ON(1);
> +	if (WARN_ON(down_read_trylock(&mm->mmap_sem)))
>  		up_read(&mm->mmap_sem);
> -	}
>  #endif
>  }
>  
> -- 
> 2.17.0
> 

-- 
Michal Hocko
SUSE Labs