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, 38 insertions, 17 deletions

diff --git a/mm/swap_state.c b/mm/swap_state.c
index 0cb36fb1f61c..8d6644c5d0cc 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -36,12 +36,12 @@ static struct backing_dev_info swap_backing_dev_info = {
         .capabilities   = BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
 };
 
-struct address_space swapper_space = {
-        .page_tree      = RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
-        .tree_lock      = __SPIN_LOCK_UNLOCKED(swapper_space.tree_lock),
-        .a_ops          = &swap_aops,
-        .i_mmap_nonlinear = LIST_HEAD_INIT(swapper_space.i_mmap_nonlinear),
-        .backing_dev_info = &swap_backing_dev_info,
+struct address_space swapper_spaces[MAX_SWAPFILES] = {
+        [0 ... MAX_SWAPFILES - 1] = {
+                .page_tree      = RADIX_TREE_INIT(GFP_ATOMIC|__GFP_NOWARN),
+                .a_ops          = &swap_aops,
+                .backing_dev_info = &swap_backing_dev_info,
+        }
 };
 
 #define INC_CACHE_INFO(x)       do { swap_cache_info.x++; } while (0)
@@ -53,9 +53,19 @@ static struct {
         unsigned long find_total;
 } swap_cache_info;
 
+unsigned long total_swapcache_pages(void)
+{
+        int i;
+        unsigned long ret = 0;
+
+        for (i = 0; i < MAX_SWAPFILES; i++)
+                ret += swapper_spaces[i].nrpages;
+        return ret;
+}
+
 void show_swap_cache_info(void)
 {
-        printk("%lu pages in swap cache\n", total_swapcache_pages);
+        printk("%lu pages in swap cache\n", total_swapcache_pages());
         printk("Swap cache stats: add %lu, delete %lu, find %lu/%lu\n",
                 swap_cache_info.add_total, swap_cache_info.del_total,
                 swap_cache_info.find_success, swap_cache_info.find_total);
@@ -70,6 +80,7 @@ void show_swap_cache_info(void)
 static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
 {
         int error;
+        struct address_space *address_space;
 
         VM_BUG_ON(!PageLocked(page));
         VM_BUG_ON(PageSwapCache(page));
@@ -79,14 +90,16 @@ static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
         SetPageSwapCache(page);
         set_page_private(page, entry.val);
 
-        spin_lock_irq(&swapper_space.tree_lock);
-        error = radix_tree_insert(&swapper_space.page_tree, entry.val, page);
+        address_space = swap_address_space(entry);
+        spin_lock_irq(&address_space->tree_lock);
+        error = radix_tree_insert(&address_space->page_tree,
+                                        entry.val, page);
         if (likely(!error)) {
-                total_swapcache_pages++;
+                address_space->nrpages++;
                 __inc_zone_page_state(page, NR_FILE_PAGES);
                 INC_CACHE_INFO(add_total);
         }
-        spin_unlock_irq(&swapper_space.tree_lock);
+        spin_unlock_irq(&address_space->tree_lock);
 
         if (unlikely(error)) {
                 /*
@@ -122,14 +135,19 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
  */
 void __delete_from_swap_cache(struct page *page)
 {
+        swp_entry_t entry;
+        struct address_space *address_space;
+
         VM_BUG_ON(!PageLocked(page));
         VM_BUG_ON(!PageSwapCache(page));
         VM_BUG_ON(PageWriteback(page));
 
-        radix_tree_delete(&swapper_space.page_tree, page_private(page));
+        entry.val = page_private(page);
+        address_space = swap_address_space(entry);
+        radix_tree_delete(&address_space->page_tree, page_private(page));
         set_page_private(page, 0);
         ClearPageSwapCache(page);
-        total_swapcache_pages--;
+        address_space->nrpages--;
         __dec_zone_page_state(page, NR_FILE_PAGES);
         INC_CACHE_INFO(del_total);
 }
@@ -195,12 +213,14 @@ int add_to_swap(struct page *page)
 void delete_from_swap_cache(struct page *page)
 {
         swp_entry_t entry;
+        struct address_space *address_space;
 
         entry.val = page_private(page);
 
-        spin_lock_irq(&swapper_space.tree_lock);
+        address_space = swap_address_space(entry);
+        spin_lock_irq(&address_space->tree_lock);
         __delete_from_swap_cache(page);
-        spin_unlock_irq(&swapper_space.tree_lock);
+        spin_unlock_irq(&address_space->tree_lock);
 
         swapcache_free(entry, page);
         page_cache_release(page);
@@ -263,7 +283,7 @@ struct page * lookup_swap_cache(swp_entry_t entry)
 {
         struct page *page;
 
-        page = find_get_page(&swapper_space, entry.val);
+        page = find_get_page(swap_address_space(entry), entry.val);
 
         if (page)
                 INC_CACHE_INFO(find_success);
@@ -290,7 +310,8 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
                  * called after lookup_swap_cache() failed, re-calling
                  * that would confuse statistics.
                  */
-                found_page = find_get_page(&swapper_space, entry.val);
+                found_page = find_get_page(swap_address_space(entry),
+                                        entry.val);
                 if (found_page)
                         break;