[PATCH 2/3] mm: Remove unlikely() from page_mapping()

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

page_mapping() has a unlikely that the mapping has PAGE_MAPPING_ANON
set. But running the annotated branch profiler on a normal desktop
system doing vairous tasks (xchat, evolution, firefox, distcc),
it is not really that unlikely that the mapping here will have the
PAGE_MAPPING_ANON flag set:

 correct incorrect  %        Function                  File              Line
 ------- ---------  -        --------                  ----              ----
35935762 1270265395  97 page_mapping                   mm.h                 659
1306198001   143659   0 page_mapping                   mm.h                 657
203131478   121586   0 page_mapping                   mm.h                 657
 5415491     1116   0 page_mapping                   mm.h                 657
74899487     1116   0 page_mapping                   mm.h                 657
203132845      224   0 page_mapping                   mm.h                 659
 5415464       27   0 page_mapping                   mm.h                 659
   13552        0   0 page_mapping                   mm.h                 657
   13552        0   0 page_mapping                   mm.h                 659
  242630        0   0 page_mapping                   mm.h                 657
  242630        0   0 page_mapping                   mm.h                 659
74899487        0   0 page_mapping                   mm.h                 659

The page_mapping() is a static inline, which is why it shows up multiple
times.

The unlikely in page_mapping() was correct a total of 1909540379 times and
incorrect 1270533123 times, with a 39% being incorrect. With this much of an
error, it's best to simply remove the unlikely and have the compiler and
branch prediction figure this out.

Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 include/linux/mm.h |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 721f451..b064264 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -657,7 +657,7 @@ static inline struct address_space *page_mapping(struct page *page)
 	VM_BUG_ON(PageSlab(page));
 	if (unlikely(PageSwapCache(page)))
 		mapping = &swapper_space;
-	else if (unlikely((unsigned long)mapping & PAGE_MAPPING_ANON))
+	else if ((unsigned long)mapping & PAGE_MAPPING_ANON)
 		mapping = NULL;
 	return mapping;
 }
-- 
1.7.2.3


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