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

[David Hildenbrand]

On 01.12.23 20:59, Steven Rostedt wrote:
> From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
>
> 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@xxxxxxxxxx>

But

> Cc: stable@xxxxxxxxxxxxxxx

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