On 12/1/23 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
>
> 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@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
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);
