From: Rik van Riel <riel@redhat.com>

Free the swap space of already resident pages when swap space starts
getting tight, instead of only freeing up the swap space taken up by newly
swapped in pages.

This should result in the swap space of pages that remain resident in
memory being freed, allowing kswapd more chances to actually swap a page
out (instead of rotating it back onto the active list).

Signed-off-by: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/pagevec.h |    1 +
 mm/swap.c               |   18 ++++++++++++++++++
 mm/vmscan.c             |    7 +++++++
 3 files changed, 26 insertions(+)

diff -puN include/linux/pagevec.h~free-swap-space-of-reactivated-pages include/linux/pagevec.h
--- a/include/linux/pagevec.h~free-swap-space-of-reactivated-pages
+++ a/include/linux/pagevec.h
@@ -26,6 +26,7 @@ void __pagevec_free(struct pagevec *pvec
 void __pagevec_lru_add(struct pagevec *pvec);
 void __pagevec_lru_add_active(struct pagevec *pvec);
 void pagevec_strip(struct pagevec *pvec);
+void pagevec_swap_free(struct pagevec *pvec);
 unsigned pagevec_lookup(struct pagevec *pvec, struct address_space *mapping,
 		pgoff_t start, unsigned nr_pages);
 unsigned pagevec_lookup_tag(struct pagevec *pvec,
diff -puN mm/swap.c~free-swap-space-of-reactivated-pages mm/swap.c
--- a/mm/swap.c~free-swap-space-of-reactivated-pages
+++ a/mm/swap.c
@@ -420,6 +420,24 @@ void pagevec_strip(struct pagevec *pvec)
 	}
 }
 
+/*
+ * Try to free swap space from the pages in a pagevec
+ */
+void pagevec_swap_free(struct pagevec *pvec)
+{
+	int i;
+
+	for (i = 0; i < pagevec_count(pvec); i++) {
+		struct page *page = pvec->pages[i];
+
+		if (PageSwapCache(page) && !TestSetPageLocked(page)) {
+			if (PageSwapCache(page))
+				remove_exclusive_swap_page(page);
+			unlock_page(page);
+		}
+	}
+}
+
 /**
  * pagevec_lookup - gang pagecache lookup
  * @pvec:	Where the resulting pages are placed
diff -puN mm/vmscan.c~free-swap-space-of-reactivated-pages mm/vmscan.c
--- a/mm/vmscan.c~free-swap-space-of-reactivated-pages
+++ a/mm/vmscan.c
@@ -587,6 +587,9 @@ free_it:
 		continue;
 
 activate_locked:
+		/* Not a candidate for swapping, so reclaim swap space. */
+		if (PageSwapCache(page) && vm_swap_full())
+			remove_exclusive_swap_page(page);
 		SetPageActive(page);
 		pgactivate++;
 keep_locked:
@@ -889,6 +892,8 @@ force_reclaim_mapped:
 			__mod_zone_page_state(zone, NR_ACTIVE, pgmoved);
 			pgmoved = 0;
 			spin_unlock_irq(&zone->lru_lock);
+			if (vm_swap_full())
+				pagevec_swap_free(&pvec);
 			__pagevec_release(&pvec);
 			spin_lock_irq(&zone->lru_lock);
 		}
@@ -899,6 +904,8 @@ force_reclaim_mapped:
 	__count_vm_events(PGDEACTIVATE, pgdeactivate);
 	spin_unlock_irq(&zone->lru_lock);
 
+	if (vm_swap_full())
+		pagevec_swap_free(&pvec);
 	pagevec_release(&pvec);
 }
 
_