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, 3 insertions, 2 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index e97a0e5aea91..e51864e6fe8b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -79,7 +79,7 @@ __try_to_reclaim_swap(struct swap_info_struct *si, unsigned long offset)
         struct page *page;
         int ret = 0;
 
-        page = find_get_page(&swapper_space, entry.val);
+        page = find_get_page(swap_address_space(entry), entry.val);
         if (!page)
                 return 0;
         /*
@@ -699,7 +699,8 @@ int free_swap_and_cache(swp_entry_t entry)
         p = swap_info_get(entry);
         if (p) {
                 if (swap_entry_free(p, entry, 1) == SWAP_HAS_CACHE) {
-                        page = find_get_page(&swapper_space, entry.val);
+                        page = find_get_page(swap_address_space(entry),
+                                                entry.val);
                         if (page && !trylock_page(page)) {
                                 page_cache_release(page);
                                 page = NULL;