[PATCH] fs/namespace: eliminate unnecessary mount counting

[Hao Lee <haolee.swjtu@gmail.com>]

propagate_one() counts the number of propagated mounts in each
propagation. We can count them in advance and use the number in
subsequent propagation.

Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>
---
 fs/namespace.c | 27 +++++++++++++++++++--------
 fs/pnode.c     | 12 +++++++-----
 fs/pnode.h     |  5 +++--
 3 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index c6feb92209a6..5d05392854ca 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2082,18 +2082,15 @@ static int invent_group_ids(struct mount *mnt, bool recurse)
 	return 0;
 }
 
-int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
+int update_pending_mounts(struct mnt_namespace *ns, unsigned int mounts)
 {
 	unsigned int max = READ_ONCE(sysctl_mount_max);
-	unsigned int mounts = 0, old, pending, sum;
-	struct mount *p;
-
-	for (p = mnt; p; p = next_mnt(p, mnt))
-		mounts++;
+	unsigned int old, pending, sum;
 
 	old = ns->mounts;
 	pending = ns->pending_mounts;
 	sum = old + pending;
+
 	if ((old > sum) ||
 	    (pending > sum) ||
 	    (max < sum) ||
@@ -2104,6 +2101,17 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
 	return 0;
 }
 
+unsigned int count_mounts(struct mount *mnt)
+{
+	unsigned int mounts = 0;
+	struct mount *p;
+
+	for (p = mnt; p; p = next_mnt(p, mnt))
+		mounts++;
+
+	return mounts;
+}
+
 /*
  *  @source_mnt : mount tree to be attached
  *  @nd         : place the mount tree @source_mnt is attached
@@ -2178,6 +2186,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	struct mountpoint *smp;
 	struct mount *child, *p;
 	struct hlist_node *n;
+	unsigned int nr_mounts;
 	int err;
 
 	/* Preallocate a mountpoint in case the new mounts need
@@ -2187,9 +2196,10 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	if (IS_ERR(smp))
 		return PTR_ERR(smp);
 
+	nr_mounts = count_mounts(source_mnt);
 	/* Is there space to add these mounts to the mount namespace? */
 	if (!moving) {
-		err = count_mounts(ns, source_mnt);
+		err = update_pending_mounts(ns, nr_mounts);
 		if (err)
 			goto out;
 	}
@@ -2198,7 +2208,8 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 		err = invent_group_ids(source_mnt, true);
 		if (err)
 			goto out;
-		err = propagate_mnt(dest_mnt, dest_mp, source_mnt, &tree_list);
+		err = propagate_mnt(dest_mnt, dest_mp, source_mnt, nr_mounts,
+				    &tree_list);
 		lock_mount_hash();
 		if (err)
 			goto out_cleanup_ids;
diff --git a/fs/pnode.c b/fs/pnode.c
index 1106137c747a..877de718fc35 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -222,7 +222,7 @@ static inline bool peers(struct mount *m1, struct mount *m2)
 	return m1->mnt_group_id == m2->mnt_group_id && m1->mnt_group_id;
 }
 
-static int propagate_one(struct mount *m)
+static int propagate_one(struct mount *m, unsigned int nr_mounts)
 {
 	struct mount *child;
 	int type;
@@ -269,7 +269,7 @@ static int propagate_one(struct mount *m)
 	last_dest = m;
 	last_source = child;
 	hlist_add_head(&child->mnt_hash, list);
-	return count_mounts(m->mnt_ns, child);
+	return update_pending_mounts(m->mnt_ns, nr_mounts);
 }
 
 /*
@@ -284,9 +284,11 @@ static int propagate_one(struct mount *m)
  * @dest_dentry: destination dentry.
  * @source_mnt: source mount.
  * @tree_list : list of heads of trees to be attached.
+ * @nr_mounts : the number of mounts in source_mnt
  */
 int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
-		    struct mount *source_mnt, struct hlist_head *tree_list)
+		    struct mount *source_mnt, unsigned int nr_mounts,
+		    struct hlist_head *tree_list)
 {
 	struct mount *m, *n;
 	int ret = 0;
@@ -305,7 +307,7 @@ int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
 
 	/* all peers of dest_mnt, except dest_mnt itself */
 	for (n = next_peer(dest_mnt); n != dest_mnt; n = next_peer(n)) {
-		ret = propagate_one(n);
+		ret = propagate_one(n, nr_mounts);
 		if (ret)
 			goto out;
 	}
@@ -316,7 +318,7 @@ int propagate_mnt(struct mount *dest_mnt, struct mountpoint *dest_mp,
 		/* everything in that slave group */
 		n = m;
 		do {
-			ret = propagate_one(n);
+			ret = propagate_one(n, nr_mounts);
 			if (ret)
 				goto out;
 			n = next_peer(n);
diff --git a/fs/pnode.h b/fs/pnode.h
index 988f1aa9b02a..005355c0dd49 100644
--- a/fs/pnode.h
+++ b/fs/pnode.h
@@ -38,7 +38,7 @@ static inline void set_mnt_shared(struct mount *mnt)
 
 void change_mnt_propagation(struct mount *, int);
 int propagate_mnt(struct mount *, struct mountpoint *, struct mount *,
-		struct hlist_head *);
+		  unsigned int, struct hlist_head *);
 int propagate_umount(struct list_head *);
 int propagate_mount_busy(struct mount *, int);
 void propagate_mount_unlock(struct mount *);
@@ -52,5 +52,6 @@ void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp,
 struct mount *copy_tree(struct mount *, struct dentry *, int);
 bool is_path_reachable(struct mount *, struct dentry *,
 			 const struct path *root);
-int count_mounts(struct mnt_namespace *ns, struct mount *mnt);
+int update_pending_mounts(struct mnt_namespace *ns, unsigned int mounts);
+unsigned int count_mounts(struct mount *mnt);
 #endif /* _LINUX_PNODE_H */
-- 
2.34.1