linux-kernel - Re: [PATCH] mm/rmap: Fix misplaced parenthesis of a likely()

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Message-ID: <f8bb6a60-af0d-e54c-cd81-6040890b8c89@suse.cz>
Date:   Mon, 4 Dec 2023 16:28:32 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/rmap: Fix misplaced parenthesis of a likely()

On 12/1/23 20:59, Steven Rostedt wrote:
> From: Steven Rostedt (Google) <rostedt@...dmis.org>
> 
> Running my yearly branch profiler to see where likely/unlikely annotation
> may be added or removed, I discovered this:
> 
> correct incorrect  %        Function                  File              Line
>  ------- ---------  -        --------                  ----              ----
>        0   457918 100 page_try_dup_anon_rmap         rmap.h               264
> [..]
>   458021        0   0 page_try_dup_anon_rmap         rmap.h               265
> 
> I thought it was interesting that line 264 of rmap.h had a 100% incorrect
> annotation, but the line directly below it was 100% correct. Looking at the
> code:
> 
> 	if (likely(!is_device_private_page(page) &&
> 	    unlikely(page_needs_cow_for_dma(vma, page))))
> 
> It didn't make sense. The "likely()" was around the entire if statement
> (not just the "!is_device_private_page(page)"), which also included the
> "unlikely()" portion of that if condition.
> 
> If the unlikely portion is unlikely to be true, that would make the entire
> if condition unlikely to be true, so it made no sense at all to say the
> entire if condition is true.
> 
> What is more likely to be likely is just the first part of the if statement
> before the && operation. It's likely to be a misplaced parenthesis. And
> after making the if condition broken into a likely() && unlikely(), both
> now appear to be correct!
> 
> Cc: stable@...r.kernel.org
> Fixes:fb3d824d1a46c ("mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Pragmatically speaking, stable maintainers haven't been following the stable
rules for a long time, and a commit with Fixes and without Cc: stable is
often backported on the assumption people forget Cc: stable, and "Fixes:"
implies there's a bug to fix, and it's good to have bugs fixed in stable...

We have (repeatedly...) had mm extempted from this and Cc: stable is
required, which is good. So if Steven thinks there are reasons to backport,
then I'd rather let him keep the Cc: stable, instead of this later becoming
an argument to question the mm extemption again :)

> ---
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index b26fe858fd44..3c2fc291b071 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -261,8 +261,8 @@ static inline int page_try_dup_anon_rmap(struct page *page, bool compound,
>  	 * guarantee the pinned page won't be randomly replaced in the
>  	 * future on write faults.
>  	 */
> -	if (likely(!is_device_private_page(page) &&
> -	    unlikely(page_needs_cow_for_dma(vma, page))))
> +	if (likely(!is_device_private_page(page)) &&
> +	    unlikely(page_needs_cow_for_dma(vma, page)))
>  		return -EBUSY;
>  
>  	ClearPageAnonExclusive(page);