Index: sys/kern/subr_blist.c
===================================================================
--- sys/kern/subr_blist.c
+++ sys/kern/subr_blist.c
@@ -295,7 +295,7 @@
  *		     not be allocated.
  */
 daddr_t
-blist_alloc(blist_t bl, int *count, int maxcount)
+blist_alloc(blist_t bl, daddr_t *io_cursor, int *count, int maxcount)
 {
 	daddr_t blk, cursor;
 
@@ -310,14 +310,14 @@
 	 * non-zero.  When the cursor is zero, an allocation failure will
 	 * stop further iterations.
 	 */
-	for (cursor = bl->bl_cursor;; cursor = 0) {
+	for (cursor = *io_cursor;; cursor = 0) {
 		blk = blst_meta_alloc(bl->bl_root, cursor, count, maxcount,
 		    bl->bl_radix);
 		if (blk != SWAPBLK_NONE) {
 			bl->bl_avail -= *count;
-			bl->bl_cursor = blk + *count;
-			if (bl->bl_cursor == bl->bl_blocks)
-				bl->bl_cursor = 0;
+			*io_cursor = blk + *count;
+			if (*io_cursor == bl->bl_blocks)
+				*io_cursor = 0;
 			return (blk);
 		}
 		if (cursor == 0)
@@ -404,8 +404,7 @@
 void
 blist_print(blist_t bl)
 {
-	printf("BLIST avail = %jd, cursor = %08jx {\n",
-	    (uintmax_t)bl->bl_avail, (uintmax_t)bl->bl_cursor);
+	printf("BLIST avail = %jd {\n", (uintmax_t)bl->bl_avail);
 
 	if (bl->bl_root->bm_bitmap != 0)
 		blst_radix_print(bl->bl_root, 0, bl->bl_radix, 4);
Index: sys/sys/blist.h
===================================================================
--- sys/sys/blist.h
+++ sys/sys/blist.h
@@ -81,7 +81,6 @@
 	daddr_t		bl_blocks;	/* area of coverage		*/
 	daddr_t		bl_avail;	/* # available blocks		*/
 	u_daddr_t	bl_radix;	/* coverage radix		*/
-	daddr_t		bl_cursor;	/* next-fit search starts at	*/
 	blmeta_t	bl_root[1];	/* root of radix tree		*/
 } *blist_t;
 
@@ -92,7 +91,7 @@
 
 struct sbuf;
 
-daddr_t	blist_alloc(blist_t blist, int *count, int maxcount);
+daddr_t	blist_alloc(blist_t blist, daddr_t *cursor, int *count, int maxcount);
 daddr_t	blist_avail(blist_t blist);
 blist_t	blist_create(daddr_t blocks, int flags);
 void	blist_destroy(blist_t blist);
Index: sys/vm/swap_pager.h
===================================================================
--- sys/vm/swap_pager.h
+++ sys/vm/swap_pager.h
@@ -65,6 +65,8 @@
 	swblk_t	sw_first;
 	swblk_t	sw_end;
 	struct blist *sw_blist;
+	daddr_t	sw_cursor;	/* next-fit search starts at	*/
+	daddr_t sw_trimmer;	/* where to look for free space to trim */
 	TAILQ_ENTRY(swdevt)	sw_list;
 	sw_strategy_t		*sw_strategy;
 	sw_close_t		*sw_close;
Index: sys/vm/swap_pager.c
===================================================================
--- sys/vm/swap_pager.c
+++ sys/vm/swap_pager.c
@@ -737,7 +737,8 @@
 		if (sp == NULL)
 			sp = TAILQ_FIRST(&swtailq);
 		if ((sp->sw_flags & SW_CLOSING) == 0)
-			blk = blist_alloc(sp->sw_blist, &npages, mpages);
+			blk = blist_alloc(sp->sw_blist, &sp->sw_cursor,
+			    &npages, mpages);
 		if (blk != SWAPBLK_NONE)
 			break;
 		sp = TAILQ_NEXT(sp, sw_list);
@@ -800,7 +801,125 @@
 }
 
 
+static void
+swp_pager_async_trimdone(struct buf *bp)
+{
+	struct swdevt *sp;
+	daddr_t blk;
+	int npages;
+
+	sp = (struct swdevt *)bp->b_fsprivate1;
+	blk = bp->b_blkno;
+	npages = bp->b_npages;
+	uma_zfree(swwbuf_zone, bp);
+
+	mtx_lock(&swbuf_mtx);
+	if (++nsw_wcount_async == 1)
+		wakeup(&nsw_wcount_async);
+	mtx_unlock(&swbuf_mtx);
+
+	mtx_lock(&sw_dev_mtx);
+	blk -= sp->sw_first;
+	blist_free(sp->sw_blist, blk, npages);
+	mtx_unlock(&sw_dev_mtx);
+}
+
+static int trimzone_numer = 1;
+static int trimzone_denom = 8;
+static int min_trim_alloc = 1;
+static int max_trim_alloc = 32768;
 /*
+ *  The 'trim zone' is the small range of addresses just ahead of the cursor
+ *  likely to be allocated soon.  When the trimmer falls into the trim zone,
+ *  we allocate blocks with the trimmer.
+ */
+static bool
+swapdev_in_trim_zone(daddr_t start, daddr_t end, daddr_t nblks)
+{
+	return ((end + nblks - start) % nblks / trimzone_numer <
+	    nblks / trimzone_denom);
+}
+
+
+/* 	sw_dev_mtx lock is held. */
+
+static struct buf *
+swapdev_trim(struct swdevt *sp)
+{
+	struct buf *bp;
+	daddr_t blk;
+	u_long nblks;
+	int npages;
+
+	/* Quit if the cursor is too far behind the trimmer. */
+	nblks = sp->sw_nblks;
+	if (!swapdev_in_trim_zone(sp->sw_cursor, sp->sw_trimmer, nblks))
+		return (NULL);
+
+	/* Grab a (hopefully) lot of free space allocated long ago. */
+	npages = min_trim_alloc;
+	blk = blist_alloc(sp->sw_blist, &sp->sw_trimmer,
+	    &npages, max_trim_alloc);
+
+	/* Quit if there's nothing free. */
+	if (blk == SWAPBLK_NONE)
+		return (NULL);
+	if (swapdev_in_trim_zone(sp->sw_cursor, sp->sw_trimmer, nblks)) {
+		/*
+		 * Trim allocation is too close to the cursor. Either we've
+		 * wrapped around and jumped the cursor, or we've found too
+		 * little free space close to the cursor to move the trimmer
+		 * out of the trim zone.  Bail out.
+		 *
+		 */
+		blist_free(sp->sw_blist, blk, npages);
+		return (NULL);
+	}
+	blk += sp->sw_first;
+	mtx_unlock(&sw_dev_mtx);
+	CTR3(KTR_SPARE5, "swapdev_trim: cursor %p trimmer %p trimsize %d",
+	     (void*)sp->sw_cursor, (void*)sp->sw_trimmer, npages);
+	mtx_lock(&swbuf_mtx);
+	while (nsw_wcount_async == 0)
+		msleep(&nsw_wcount_async, &swbuf_mtx, PVM,
+		       "swbufa", 0);
+	nsw_wcount_async--;
+	mtx_unlock(&swbuf_mtx);
+	bp = uma_zalloc(swwbuf_zone, M_WAITOK);
+	bp->b_flags = B_ASYNC;
+	bp->b_iocmd = BIO_DELETE;
+	bp->b_rcred = crhold(thread0.td_ucred);
+	bp->b_wcred = crhold(thread0.td_ucred);
+	bp->b_bcount = PAGE_SIZE * npages;
+	bp->b_bufsize = PAGE_SIZE * npages;
+	bp->b_blkno = blk;
+	bp->b_npages = npages;
+	bp->b_iodone = swp_pager_async_trimdone;
+	bp->b_fsprivate1 = sp;
+	BUF_KERNPROC(bp);
+	return (bp);
+}
+
+/* find a swap device that needs trimming, and start trimming */
+static void
+swp_pager_trimswapspace(void)
+{
+	struct swdevt *sp;
+	struct buf *bp;
+
+	mtx_lock(&sw_dev_mtx);
+	TAILQ_FOREACH(sp, &swtailq, sw_list) {
+		bp = swapdev_trim(sp);
+		if (bp == NULL)
+			continue;
+		/* sw_dev_mtx released in swapdev_trim */
+		swp_pager_strategy(bp);
+		return;
+	}
+	mtx_unlock(&sw_dev_mtx);
+}
+
+/*
  * SWP_PAGER_FREESWAPSPACE() -	free raw swap space
  *
  *	This routine returns the specified swap blocks back to the bitmap.
@@ -921,6 +1040,7 @@
 	}
 	swp_pager_freeswapspace(s_free, n_free);
 	VM_OBJECT_WUNLOCK(object);
+	swp_pager_trimswapspace();
 	return (0);
 }
 
@@ -1388,6 +1508,7 @@
 
 		/* Get a block of swap of size up to size n. */
 		blk = swp_pager_getswapspace(&n, 4);
+		swp_pager_trimswapspace();
 		if (blk == SWAPBLK_NONE) {
 			for (j = 0; j < n; ++j)
 				rtvals[i+j] = VM_PAGER_FAIL;
@@ -2310,6 +2431,8 @@
 	sp->sw_flags = flags;
 
 	sp->sw_blist = blist_create(nblks, M_WAITOK);
+	sp->sw_cursor = 0;
+	sp->sw_trimmer = 0;
 	/*
 	 * Do not free the first two block in order to avoid overwriting
 	 * any bsd label at the front of the partition