swapfile: rearrange scan and swap_info

Before making functional changes, rearrange scan_swap_map() to simplify
subsequent diffs.  Actually, there is one functional change in there:
leave cluster_nr negative while scanning for a new cluster - resetting it
early increased the likelihood that when we have difficulty finding a free
cluster, another task may come in and try doing exactly the same - just a
waste of cpu.

Before making functional changes, rearrange struct swap_info_struct
slightly: flags will be needed as an unsigned long (for wait_on_bit), next
is a good int to pair with prio, old_block_size is uninteresting so shift
it to the end.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 41 insertions, 33 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 410c8e473727..9cabb8b21aba 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -133,14 +133,14 @@ enum {
  * The in-memory structure used to track swap areas.
  */
 struct swap_info_struct {
-        unsigned int flags;
+        unsigned long flags;
         int prio;                       /* swap priority */
+        int next;                       /* next entry on swap list */
         struct file *swap_file;
         struct block_device *bdev;
         struct list_head extent_list;
         struct swap_extent *curr_swap_extent;
-        unsigned old_block_size;
-        unsigned short * swap_map;
+        unsigned short *swap_map;
         unsigned int lowest_bit;
         unsigned int highest_bit;
         unsigned int cluster_next;
@@ -148,7 +148,7 @@ struct swap_info_struct {
         unsigned int pages;
         unsigned int max;
         unsigned int inuse_pages;
-        int next;                       /* next entry on swap list */
+        unsigned int old_block_size;
 };
 
 struct swap_list_t {
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 85ff603385c3..4d9855f86e7d 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -89,7 +89,8 @@ void swap_unplug_io_fn(struct backing_dev_info *unused_bdi, struct page *page)
 
 static inline unsigned long scan_swap_map(struct swap_info_struct *si)
 {
-        unsigned long offset, last_in_cluster;
+        unsigned long offset;
+        unsigned long last_in_cluster;
         int latency_ration = LATENCY_LIMIT;
 
         /*
@@ -103,10 +104,13 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
          */
 
         si->flags += SWP_SCANNING;
-        if (unlikely(!si->cluster_nr)) {
-                si->cluster_nr = SWAPFILE_CLUSTER - 1;
-                if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER)
-                        goto lowest;
+        offset = si->cluster_next;
+
+        if (unlikely(!si->cluster_nr--)) {
+                if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER) {
+                        si->cluster_nr = SWAPFILE_CLUSTER - 1;
+                        goto checks;
+                }
                 spin_unlock(&swap_lock);
 
                 offset = si->lowest_bit;
@@ -118,43 +122,47 @@ static inline unsigned long scan_swap_map(struct swap_info_struct *si)
                                 last_in_cluster = offset + SWAPFILE_CLUSTER;
                         else if (offset == last_in_cluster) {
                                 spin_lock(&swap_lock);
-                                si->cluster_next = offset-SWAPFILE_CLUSTER+1;
-                                goto cluster;
+                                offset -= SWAPFILE_CLUSTER - 1;
+                                si->cluster_next = offset;
+                                si->cluster_nr = SWAPFILE_CLUSTER - 1;
+                                goto checks;
                         }
                         if (unlikely(--latency_ration < 0)) {
                                 cond_resched();
                                 latency_ration = LATENCY_LIMIT;
                         }
                 }
+
+                offset = si->lowest_bit;
                 spin_lock(&swap_lock);
-                goto lowest;
+                si->cluster_nr = SWAPFILE_CLUSTER - 1;
         }
 
-        si->cluster_nr--;
-cluster:
-        offset = si->cluster_next;
-        if (offset > si->highest_bit)
-lowest:         offset = si->lowest_bit;
-checks: if (!(si->flags & SWP_WRITEOK))
+checks:
+        if (!(si->flags & SWP_WRITEOK))
                 goto no_page;
         if (!si->highest_bit)
                 goto no_page;
-        if (!si->swap_map[offset]) {
-                if (offset == si->lowest_bit)
-                        si->lowest_bit++;
-                if (offset == si->highest_bit)
-                        si->highest_bit--;
-                si->inuse_pages++;
-                if (si->inuse_pages == si->pages) {
-                        si->lowest_bit = si->max;
-                        si->highest_bit = 0;
-                }
-                si->swap_map[offset] = 1;
-                si->cluster_next = offset + 1;
-                si->flags -= SWP_SCANNING;
-                return offset;
+        if (offset > si->highest_bit)
+                offset = si->lowest_bit;
+        if (si->swap_map[offset])
+                goto scan;
+
+        if (offset == si->lowest_bit)
+                si->lowest_bit++;
+        if (offset == si->highest_bit)
+                si->highest_bit--;
+        si->inuse_pages++;
+        if (si->inuse_pages == si->pages) {
+                si->lowest_bit = si->max;
+                si->highest_bit = 0;
         }
+        si->swap_map[offset] = 1;
+        si->cluster_next = offset + 1;
+        si->flags -= SWP_SCANNING;
+        return offset;
 
+scan:
         spin_unlock(&swap_lock);
         while (++offset <= si->highest_bit) {
                 if (!si->swap_map[offset]) {
@@ -167,7 +175,7 @@ checks:	if (!(si->flags & SWP_WRITEOK))
                 }
         }
         spin_lock(&swap_lock);
-        goto lowest;
+        goto checks;
 
 no_page:
         si->flags -= SWP_SCANNING;