swap: make each swap partition have one address_space

When I use several fast SSD to do swap, swapper_space.tree_lock is
heavily contended.  This makes each swap partition have one
address_space to reduce the lock contention.  There is an array of
address_space for swap.  The swap entry type is the index to the array.

In my test with 3 SSD, this increases the swapout throughput 20%.

[akpm@linux-foundation.org: revert unneeded change to  __add_to_swap_cache]
Signed-off-by: Shaohua Li <shli@fusionio.com>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/mm/swap.c b/mm/swap.c
index 6310dc2008ff..8a529a01e8fc 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -855,9 +855,14 @@ EXPORT_SYMBOL(pagevec_lookup_tag);
 void __init swap_setup(void)
 {
         unsigned long megs = totalram_pages >> (20 - PAGE_SHIFT);
-
 #ifdef CONFIG_SWAP
-        bdi_init(swapper_space.backing_dev_info);
+        int i;
+
+        bdi_init(swapper_spaces[0].backing_dev_info);
+        for (i = 0; i < MAX_SWAPFILES; i++) {
+                spin_lock_init(&swapper_spaces[i].tree_lock);
+                INIT_LIST_HEAD(&swapper_spaces[i].i_mmap_nonlinear);
+        }
 #endif
 
         /* Use a smaller cluster for small-memory machines */