[PATCH] ceph: eliminate the recursion when rebuilding the snap context

[xiubli@xxxxxxxxxx]

From: Xiubo Li <xiubli@xxxxxxxxxx>

Use a list instead of recuresion to avoid possible stack overflow.

Signed-off-by: Xiubo Li <xiubli@xxxxxxxxxx>
---
 fs/ceph/snap.c  | 44 +++++++++++++++++++++++++++++++++++---------
 fs/ceph/super.h |  2 ++
 2 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/fs/ceph/snap.c b/fs/ceph/snap.c
index 6939307d41cb..808add7dca9e 100644
--- a/fs/ceph/snap.c
+++ b/fs/ceph/snap.c
@@ -319,7 +319,8 @@ static int cmpu64_rev(const void *a, const void *b)
  * build the snap context for a given realm.
  */
 static int build_snap_context(struct ceph_snap_realm *realm,
-			      struct list_head* dirty_realms)
+			      struct list_head *realm_queue,
+			      struct list_head *dirty_realms)
 {
 	struct ceph_snap_realm *parent = realm->parent;
 	struct ceph_snap_context *snapc;
@@ -333,9 +334,9 @@ static int build_snap_context(struct ceph_snap_realm *realm,
 	 */
 	if (parent) {
 		if (!parent->cached_context) {
-			err = build_snap_context(parent, dirty_realms);
-			if (err)
-				goto fail;
+			/* add to the queue head */
+			list_add(&parent->rebuild_item, realm_queue);
+			return 1;
 		}
 		num += parent->cached_context->num_snaps;
 	}
@@ -418,13 +419,38 @@ static int build_snap_context(struct ceph_snap_realm *realm,
 static void rebuild_snap_realms(struct ceph_snap_realm *realm,
 				struct list_head *dirty_realms)
 {
-	struct ceph_snap_realm *child;
+	LIST_HEAD(realm_queue);
+	int last = 0;
 
-	dout("%s %llx %p\n", __func__, realm->ino, realm);
-	build_snap_context(realm, dirty_realms);
+	list_add_tail(&realm->rebuild_item, &realm_queue);
+
+	while (!list_empty(&realm_queue)) {
+		struct ceph_snap_realm *_realm, *child;
+
+		/*
+		 * If the last building failed dues to memory
+		 * issue, just empty the realm_queue and return
+		 * to avoid infinite loop.
+		 */
+		if (last < 0) {
+			list_del(&_realm->rebuild_item);
+			continue;
+		}
+
+		_realm = list_first_entry(&realm_queue,
+					  struct ceph_snap_realm,
+					  rebuild_item);
+		last = build_snap_context(_realm, &realm_queue, dirty_realms);
+		dout("%s %llx %p, %s\n", __func__, _realm->ino, _realm,
+		     last > 0 ? "is deferred" : !last ? "succeeded" : "failed");
 
-	list_for_each_entry(child, &realm->children, child_item)
-		rebuild_snap_realms(child, dirty_realms);
+		list_for_each_entry(child, &_realm->children, child_item)
+			list_add_tail(&child->rebuild_item, &realm_queue);
+
+		/* last == 1 means need to build parent first */
+		if (last <= 0)
+			list_del(&_realm->rebuild_item);
+	}
 }
 
 
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index a17bd01a8957..baac800a6d11 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -885,6 +885,8 @@ struct ceph_snap_realm {
 
 	struct list_head dirty_item;     /* if realm needs new context */
 
+	struct list_head rebuild_item;   /* rebuild snap realms _downward_ in hierarchy */
+
 	/* the current set of snaps for this realm */
 	struct ceph_snap_context *cached_context;
 
-- 
2.27.0