From: Matthew Dillon <dillon@apollo.backplane.com>
Date: Sun, 3 Mar 2013 08:15:42 +0000 (-0800)
Subject: kernel - Refactor swapoff scan code
X-Git-Tag: v3.4.0rc~165
X-Git-Url: https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff_plain/427e1a99f84ec99acaaeecf046ac9fb94b9d4443?hp=3b47bfc4f83363f5a473ab71e6219ad84ebc6b77

kernel - Refactor swapoff scan code

* Refactor the scan code to be far more optimal.  The old scan was
  restarting the entire VM object scan on each swap block removed
  from the swap device being turned off.

* Scan twice before giving up to try to catch any elements which
  are in-transition.
---

diff --git a/sys/vm/swap_pager.c b/sys/vm/swap_pager.c
index 2883d6ae36..0a55b4e57d 100644
--- a/sys/vm/swap_pager.c
+++ b/sys/vm/swap_pager.c
@@ -146,6 +146,11 @@ struct swfreeinfo {
 	vm_pindex_t	endi;	/* inclusive */
 };
 
+struct swswapoffinfo {
+	vm_object_t	object;
+	int		devidx;
+};
+
 /*
  * vm_swap_size is in page-sized chunks now.  It was DEV_BSIZE'd chunks
  * in the old system.
@@ -1921,6 +1926,7 @@ swp_pager_async_iodone(struct bio *bio)
 
 /*
  * Fault-in a potentially swapped page and remove the swap reference.
+ * (used by swapoff code)
  *
  * object must be held.
  */
@@ -1962,38 +1968,50 @@ swp_pager_fault_page(vm_object_t object, vm_pindex_t pindex)
 	}
 }
 
+/*
+ * This removes all swap blocks related to a particular device.  We have
+ * to be careful of ripups during the scan.
+ */
+static int swp_pager_swapoff_callback(struct swblock *swap, void *data);
+
 int
 swap_pager_swapoff(int devidx)
 {
+	struct vm_object marker;
 	vm_object_t object;
-	struct swblock *swap;
-	swblk_t v;
-	int i;
+	struct swswapoffinfo info;
+
+	bzero(&marker, sizeof(marker));
+	marker.type = OBJT_MARKER;
 
 	lwkt_gettoken(&vmobj_token);
-rescan:
-	TAILQ_FOREACH(object, &vm_object_list, object_list) {
+	TAILQ_INSERT_HEAD(&vm_object_list, &marker, object_list);
+
+	while ((object = TAILQ_NEXT(&marker, object_list)) != NULL) {
+		if (object->type == OBJT_MARKER)
+			goto skip;
 		if (object->type != OBJT_SWAP && object->type != OBJT_VNODE)
-			continue;
+			goto skip;
 		vm_object_hold(object);
-		if (object->type == OBJT_SWAP || object->type == OBJT_VNODE) {
-			RB_FOREACH(swap,
-				   swblock_rb_tree, &object->swblock_root) {
-				for (i = 0; i < SWAP_META_PAGES; ++i) {
-					v = swap->swb_pages[i];
-					if (v != SWAPBLK_NONE &&
-					    BLK2DEVIDX(v) == devidx) {
-						swp_pager_fault_page(
-						    object,
-						    swap->swb_index + i);
-						vm_object_drop(object);
-						goto rescan;
-					}
-				}
-			}
+		if (object->type != OBJT_SWAP && object->type != OBJT_VNODE) {
+			vm_object_drop(object);
+			goto skip;
 		}
+		info.object = object;
+		info.devidx = devidx;
+		swblock_rb_tree_RB_SCAN(&object->swblock_root,
+					NULL,
+					swp_pager_swapoff_callback,
+					&info);
 		vm_object_drop(object);
+skip:
+		if (object == TAILQ_NEXT(&marker, object_list)) {
+			TAILQ_REMOVE(&vm_object_list, &marker, object_list);
+			TAILQ_INSERT_AFTER(&vm_object_list, object,
+					   &marker, object_list);
+		}
 	}
+	TAILQ_REMOVE(&vm_object_list, &marker, object_list);
 	lwkt_reltoken(&vmobj_token);
 
 	/*
@@ -2007,6 +2025,42 @@ rescan:
 		return (0);
 }
 
+static
+int
+swp_pager_swapoff_callback(struct swblock *swap, void *data)
+{
+	struct swswapoffinfo *info = data;
+	vm_object_t object = info->object;
+	vm_pindex_t index;
+	swblk_t v;
+	int i;
+
+	index = swap->swb_index;
+	for (i = 0; i < SWAP_META_PAGES; ++i) {
+		/*
+		 * Make sure we don't race a dying object.  This will
+		 * kill the scan of the object's swap blocks entirely.
+		 */
+		if (object->flags & OBJ_DEAD)
+			return(-1);
+
+		/*
+		 * Fault the page, which can obviously block.  If the swap
+		 * structure disappears break out.
+		 */
+		v = swap->swb_pages[i];
+		if (v != SWAPBLK_NONE && BLK2DEVIDX(v) == info->devidx) {
+			swp_pager_fault_page(object, swap->swb_index + i);
+			/* swap ptr might go away */
+			if (RB_LOOKUP(swblock_rb_tree,
+				      &object->swblock_root, index) != swap) {
+				break;
+			}
+		}
+	}
+	return(0);
+}
+
 /************************************************************************
  *				SWAP META DATA 				*
  ************************************************************************
diff --git a/sys/vm/vm_swap.c b/sys/vm/vm_swap.c
index 2a0a64ffa7..1a7ecef906 100644
--- a/sys/vm/vm_swap.c
+++ b/sys/vm/vm_swap.c
@@ -538,7 +538,7 @@ swapoff_one(int index)
 	/*
 	 * Page in the contents of the device and close it.
 	 */
-	if (swap_pager_swapoff(index)) {
+	if (swap_pager_swapoff(index) && swap_pager_swapoff(index)) {
 		mtx_unlock(&swap_mtx);
 		return (EINTR);
 	}