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/mincore.c b/mm/mincore.c
index 936b4cee8cb1..da2be56a7b8f 100644
--- a/mm/mincore.c
+++ b/mm/mincore.c
@@ -75,7 +75,7 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff)
         /* shmem/tmpfs may return swap: account for swapcache page too. */
         if (radix_tree_exceptional_entry(page)) {
                 swp_entry_t swap = radix_to_swp_entry(page);
-                page = find_get_page(&swapper_space, swap.val);
+                page = find_get_page(swap_address_space(swap), swap.val);
         }
 #endif
         if (page) {
@@ -135,7 +135,8 @@ static void mincore_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
                         } else {
 #ifdef CONFIG_SWAP
                                 pgoff = entry.val;
-                                *vec = mincore_page(&swapper_space, pgoff);
+                                *vec = mincore_page(swap_address_space(entry),
+                                        pgoff);
 #else
                                 WARN_ON(1);
                                 *vec = 1;