kerndat: Make fs-virtualized check report yes/no

Right now it returns the whole struct stat which is excessive.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/kerndat.h

@@ -33,6 +33,12 @@ enum {
 	KERNDAT_FS_STAT_MAX
 };
 
-extern struct stat *kerndat_get_fs_stat(unsigned int which);
+/*
+ * Check whether the fs @which with kdevice @kdev
+ * is the same as host's. If yes, this means that
+ * the fs mount is shared with host, if no -- it's
+ * a new (likely virtuzlized) fs instance.
+ */
+extern int kerndat_fs_virtualized(unsigned int which, u32 kdev);
 
 #endif /* __CR_KERNDAT_H__ */

kerndat.c

@@ -57,7 +57,7 @@ static int kerndat_get_shmemdev(void)
 	return 0;
 }
 
-struct stat *kerndat_get_fs_stat(unsigned int which)
+static struct stat *kerndat_get_fs_stat(unsigned int which)
 {
 	static struct {
 		const char	*name;
@@ -103,6 +103,17 @@ struct stat *kerndat_get_fs_stat(unsigned int which)
 	return &kstat[which].st;
 }
 
+int kerndat_fs_virtualized(unsigned int which, u32 kdev)
+{
+	struct stat *host_fs_stat;
+
+	host_fs_stat = kerndat_get_fs_stat(which);
+	if (host_fs_stat == NULL)
+		return -1;
+
+	return (kdev_to_odev(kdev) == host_fs_stat->st_dev) ? 0 : 1;
+}
+
 /*
  * Check whether pagemap reports soft dirty bit. Kernel has
  * this functionality under CONFIG_MEM_SOFT_DIRTY option.

mount.c

@@ -690,15 +690,17 @@ static int attach_option(struct mount_info *pm, char *opt)
 /* Is it mounted w or w/o the newinstance option */
 static int devpts_parse(struct mount_info *pm)
 {
-	struct stat *host_st;
-
-	host_st = kerndat_get_fs_stat(KERNDAT_FS_STAT_DEVPTS);
-	if (host_st == NULL)
-		return -1;
+	int ret;
 
-	if (host_st->st_dev == kdev_to_odev(pm->s_dev))
-		return 0;
+	ret = kerndat_fs_virtualized(KERNDAT_FS_STAT_DEVPTS, pm->s_dev);
+	if (ret <= 0)
+		return ret;
 
+	/*
+	 * Kernel hides this option, but if the fs instance
+	 * is new (virtualized) we know that it was created
+	 * with -o newinstance.
+	 */
 	return attach_option(pm, "newinstance");
 }
 
@@ -745,15 +747,7 @@ out:
 
 static bool rt_detmpfs_match(struct mount_info *pm)
 {
-	struct stat *host_st;
-
-	host_st = kerndat_get_fs_stat(KERNDAT_FS_STAT_DEVTMPFS);
-	if (host_st) {
-		if (host_st->st_dev == kdev_to_odev(pm->s_dev))
-			return true;
-	}
-
-	return false;
+	return kerndat_fs_virtualized(KERNDAT_FS_STAT_DEVTMPFS, pm->s_dev) == 0;
 }
 
 static int devtmpfs_dump(struct mount_info *pm)