Re: [PATCH 2/3] mm: thp: microoptimize compound_mapcount()

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Alex Williamson <alex.williamson@redhat.com>
Subject: Re: [PATCH 2/3] mm: thp: microoptimize compound_mapcount()
Date: Fri, 6 May 2016 20:33:30 +0300	[thread overview]
Message-ID: <20160506173330.GA9879@node.shutemov.name> (raw)
In-Reply-To: <1462547040-1737-3-git-send-email-aarcange@redhat.com>

On Fri, May 06, 2016 at 05:03:59PM +0200, Andrea Arcangeli wrote:
> compound_mapcount() is only called after PageCompound() has already
> been checked by the caller, so there's no point to check it again. Gcc
> may optimize it away too because it's inline but this will remove the
> runtime check for sure and add it'll add an assert instead.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

> ---
>  include/linux/mm.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 263f229..726ba80 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -471,8 +471,7 @@ static inline atomic_t *compound_mapcount_ptr(struct page *page)
>  
>  static inline int compound_mapcount(struct page *page)
>  {
> -	if (!PageCompound(page))
> -		return 0;
> +	VM_BUG_ON_PAGE(!PageCompound(page), page);
>  	page = compound_head(page);
>  	return atomic_read(compound_mapcount_ptr(page)) + 1;
>  }

-- 
 Kirill A. Shutemov