Re: [RFC][PATCH 05/10] mm: Remove likely() from grab_cache_page_write_begin()

Steven Rostedt
2010-12-07T02:24:13Z

[ Resending to Nick's real email address ]

From: Steven Rostedt <srostedt@redhat.com>

Running the annotated branch profiler on a box doing average work
(firefox, evolution, xchat, distcc farm), the likely() used in
grab_cache_page_write_begin() was incorrect most of the time:

correct incorrect % Function File Line
------- --------- - -------- ---- ----
1924262 71332401 97 grab_cache_page_write_begin filemap.c 2206

Adding a trace_printk() and running the function tracer limited to
just this function I can see:

gconfd-2-2696 [000] 4467.268935: grab_cache_page_write_begin: page= (null) mapping=ffff8800676a9460 index=7
gconfd-2-2696 [000] 4467.268946: grab_cache_page_write_begin <-ext3_write_begin
gconfd-2-2696 [000] 4467.268947: grab_cache_page_write_begin: page= (null) mapping=ffff8800676a9460 index=8
gconfd-2-2696 [000] 4467.268959: grab_cache_page_write_begin <-ext3_write_begin
gconfd-2-2696 [000] 4467.268960: grab_cache_page_write_begin: page= (null) mapping=ffff8800676a9460 index=9
gconfd-2-2696 [000] 4467.268972: grab_cache_page_write_begin <-ext3_write_begin
gconfd-2-2696 [000] 4467.268973: grab_cache_page_write_begin: page= (null) mapping=ffff8800676a9460 index=10
gconfd-2-2696 [000] 4467.268991: grab_cache_page_write_begin <-ext3_write_begin
gconfd-2-2696 [000] 4467.268992: grab_cache_page_write_begin: page= (null) mapping=ffff8800676a9460 index=11
gconfd-2-2696 [000] 4467.269005: grab_cache_page_write_begin <-ext3_write_begin

Which shows that a lot of calls from ext3_write_begin will result in
the page returned by "find_lock_page" will be NULL.

Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
mm/filemap.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index ea89840..d557fe7 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2218,7 +2218,7 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
gfp_notmask = __GFP_FS;
repeat:
page = find_lock_page(mapping, index);
- if (likely(page))
+ if (page)
return page;

page = __page_cache_alloc(mapping_gfp_mask(mapping) & ~gfp_notmask);