[PATCH] swap: swap extent list is ordered

There are several comments that swap's extent_list.prev points to the lowest
extent: that's not so, it's extent_list.next which points to it, as you'd
expect.  And a couple of loops in add_swap_extent which go all the way through
the list, when they should just add to the other end.

Fix those up, and let map_swap_page search the list forwards: profiles shows
it to be twice as quick that way - because prefetch works better on how the
structs are typically kmalloc'ed?  or because usually more is written to than
read from swap, and swap is allocated ascendingly?

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 9 insertions, 20 deletions

diff --git a/include/linux/swap.h b/include/linux/swap.h
index bfe3e763ccf2..38f288475e67 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -116,8 +116,6 @@ enum {
 
 /*
  * The in-memory structure used to track swap areas.
- * extent_list.prev points at the lowest-index extent.  That list is
- * sorted.
  */
 struct swap_info_struct {
         unsigned int flags;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4b39e9501d44..73521d39e985 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -832,9 +832,9 @@ sector_t map_swap_page(struct swap_info_struct *sis, pgoff_t offset)
                                 offset < (se->start_page + se->nr_pages)) {
                         return se->start_block + (offset - se->start_page);
                 }
-                lh = se->list.prev;
+                lh = se->list.next;
                 if (lh == &sis->extent_list)
-                        lh = lh->prev;
+                        lh = lh->next;
                 se = list_entry(lh, struct swap_extent, list);
                 sis->curr_swap_extent = se;
                 BUG_ON(se == start_se);         /* It *must* be present */
@@ -859,10 +859,9 @@ static void destroy_swap_extents(struct swap_info_struct *sis)
 
 /*
  * Add a block range (and the corresponding page range) into this swapdev's
- * extent list.  The extent list is kept sorted in block order.
+ * extent list.  The extent list is kept sorted in page order.
  *
- * This function rather assumes that it is called in ascending sector_t order.
- * It doesn't look for extent coalescing opportunities.
+ * This function rather assumes that it is called in ascending page order.
  */
 static int
 add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
@@ -872,16 +871,15 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
         struct swap_extent *new_se;
         struct list_head *lh;
 
-        lh = sis->extent_list.next;     /* The highest-addressed block */
-        while (lh != &sis->extent_list) {
+        lh = sis->extent_list.prev;     /* The highest page extent */
+        if (lh != &sis->extent_list) {
                 se = list_entry(lh, struct swap_extent, list);
-                if (se->start_block + se->nr_pages == start_block &&
-                    se->start_page  + se->nr_pages == start_page) {
+                BUG_ON(se->start_page + se->nr_pages != start_page);
+                if (se->start_block + se->nr_pages == start_block) {
                         /* Merge it */
                         se->nr_pages += nr_pages;
                         return 0;
                 }
-                lh = lh->next;
         }
 
         /*
@@ -894,14 +892,7 @@ add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
         new_se->nr_pages = nr_pages;
         new_se->start_block = start_block;
 
-        lh = sis->extent_list.prev;     /* The lowest block */
-        while (lh != &sis->extent_list) {
-                se = list_entry(lh, struct swap_extent, list);
-                if (se->start_block > start_block)
-                        break;
-                lh = lh->prev;
-        }
-        list_add_tail(&new_se->list, lh);
+        list_add_tail(&new_se->list, &sis->extent_list);
         sis->nr_extents++;
         return 0;
 }