mm: remove incorrect swap_count() from try_to_unuse()

commit 32c5fc10e79a7053ac5728b01a0bff55cbcb9d49 upstream.

In try_to_unuse(), swcount is a local copy of *swap_map, including the
SWAP_HAS_CACHE bit; but a wrong comparison against swap_count(*swap_map),
which masks off the SWAP_HAS_CACHE bit, succeeded where it should fail.

That had the effect of resetting the mm from which to start searching
for the next swap page, to an irrelevant mm instead of to an mm in which
this swap page had been found: which may increase search time by ~20%.
But we're used to swapoff being slow, so never noticed the slowdown.

Remove that one spurious use of swap_count(): Bo Liu thought it merely
redundant, Hugh rewrote the description since it was measurably wrong.

Signed-off-by: Bo Liu <bo-liu@hotmail.com>
Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 8ffdc0d23c536fe04779a9c559af68f6d9470997..b47ccd77f0504c99205dc2ec848bb777de1705c4 100644 (file)
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1149,8 +1149,7 @@ static int try_to_unuse(unsigned int type)
                                } else
                                        retval = unuse_mm(mm, entry, page);
 
-                               if (set_start_mm &&
-                                   swap_count(*swap_map) < swcount) {
+                               if (set_start_mm && *swap_map < swcount) {
                                        mmput(new_start_mm);
                                        atomic_inc(&mm->mm_users);
                                        new_start_mm = mm;