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, 2 insertions, 2 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c878b1c69510..f85861531f22 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6307,7 +6307,7 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
          * Because lookup_swap_cache() updates some statistics counter,
          * we call find_get_page() with swapper_space directly.
          */
-        page = find_get_page(&swapper_space, ent.val);
+        page = find_get_page(swap_address_space(ent), ent.val);
         if (do_swap_account)
                 entry->val = ent.val;
 
@@ -6348,7 +6348,7 @@ static struct page *mc_handle_file_pte(struct vm_area_struct *vma,
                 swp_entry_t swap = radix_to_swp_entry(page);
                 if (do_swap_account)
                         *entry = swap;
-                page = find_get_page(&swapper_space, swap.val);
+                page = find_get_page(swap_address_space(swap), swap.val);
         }
 #endif
         return page;