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, 3 deletions

diff --git a/mm/util.c b/mm/util.c
index 16a73195a37b..ab1424dbe2e6 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -6,6 +6,7 @@
 #include <linux/sched.h>
 #include <linux/security.h>
 #include <linux/swap.h>
+#include <linux/swapops.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -389,9 +390,12 @@ struct address_space *page_mapping(struct page *page)
 
         VM_BUG_ON(PageSlab(page));
 #ifdef CONFIG_SWAP
-        if (unlikely(PageSwapCache(page)))
-                mapping = &swapper_space;
-        else
+        if (unlikely(PageSwapCache(page))) {
+                swp_entry_t entry;
+
+                entry.val = page_private(page);
+                mapping = swap_address_space(entry);
+        } else
 #endif
         if ((unsigned long)mapping & PAGE_MAPPING_ANON)
                 mapping = NULL;