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mm: remove unlikely() from page_mapping()
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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.

Signed-off-by: Steven Rostedt <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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Steven Rostedt authored and torvalds committed Jan 14, 2011
1 parent 088e546 commit e20e877
Showing 1 changed file with 1 addition and 1 deletion.
2 changes: 1 addition & 1 deletion include/linux/mm.h
Original file line number Diff line number Diff line change
Expand Up @@ -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;
}
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