reduce m_start() cost...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/mount.h

@@ -10,7 +10,7 @@ struct mnt_namespace {
 	struct user_namespace	*user_ns;
 	u64			seq;	/* Sequence number to prevent loops */
 	wait_queue_head_t poll;
-	int event;
+	u64 event;
 };
 
 struct mnt_pcp {
@@ -104,6 +104,9 @@ struct proc_mounts {
 	struct mnt_namespace *ns;
 	struct path root;
 	int (*show)(struct seq_file *, struct vfsmount *);
+	void *cached_mount;
+	u64 cached_event;
+	loff_t cached_index;
 };
 
 #define proc_mounts(p) (container_of((p), struct proc_mounts, m))

fs/namespace.c

@@ -52,7 +52,7 @@ static int __init set_mphash_entries(char *str)
 }
 __setup("mphash_entries=", set_mphash_entries);
 
-static int event;
+static u64 event;
 static DEFINE_IDA(mnt_id_ida);
 static DEFINE_IDA(mnt_group_ida);
 static DEFINE_SPINLOCK(mnt_id_lock);
@@ -1100,14 +1100,29 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	struct proc_mounts *p = proc_mounts(m);
 
 	down_read(&namespace_sem);
-	return seq_list_start(&p->ns->list, *pos);
+	if (p->cached_event == p->ns->event) {
+		void *v = p->cached_mount;
+		if (*pos == p->cached_index)
+			return v;
+		if (*pos == p->cached_index + 1) {
+			v = seq_list_next(v, &p->ns->list, &p->cached_index);
+			return p->cached_mount = v;
+		}
+	}
+
+	p->cached_event = p->ns->event;
+	p->cached_mount = seq_list_start(&p->ns->list, *pos);
+	p->cached_index = *pos;
+	return p->cached_mount;
 }
 
 static void *m_next(struct seq_file *m, void *v, loff_t *pos)
 {
 	struct proc_mounts *p = proc_mounts(m);
 
-	return seq_list_next(v, &p->ns->list, pos);
+	p->cached_mount = seq_list_next(v, &p->ns->list, pos);
+	p->cached_index = *pos;
+	return p->cached_mount;
 }
 
 static void m_stop(struct seq_file *m, void *v)

fs/proc_namespace.c

@@ -267,6 +267,7 @@ static int mounts_open_common(struct inode *inode, struct file *file,
 	p->root = root;
 	p->m.poll_event = ns->event;
 	p->show = show;
+	p->cached_event = ~0ULL;
 
 	return 0;