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

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Message-ID: <2d691c11-fa97-4f56-a4c4-c7f466c81d3a@redhat.com>
Date:   Fri, 1 Dec 2023 21:06:22 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm/rmap: Fix misplaced parenthesis of a likely()

On 01.12.23 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
> 

That looks like a handy tool!

> 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.

Yes, that was clearly misplaced.

> 
> 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!
> 

Reviewed-by: David Hildenbrand <david@...hat.com>

But

> Cc: stable@...r.kernel.org

stable, really? Why?

> Fixes:fb3d824d1a46c ("mm/rmap: split page_dup_rmap() into page_dup_file_rmap() and page_try_dup_anon_rmap()")

and does it even fix a real bug?

-- 
Cheers,

David / dhildenb