mount-util: Iterate mountinfo backwards when unmounting

Submounts will always be located further in the mountinfo file, so
when we're unmounting, iterating backwards is likely to be more
efficient than iterating forwards. It'll also reduce the amount of
EBUSY debug logging we'll get since we'll stop trying to unmount
parent mounts with submounts which will always fail with EBUSY.

src/fstab-generator/fstab-generator.c

@@ -1039,7 +1039,7 @@ static int parse_fstab(bool prefix_sysroot) {
 
         log_debug("Parsing %s...", fstab);
 
-        r = libmount_parse_full(fstab, /* source = */ NULL, &table, &iter);
+        r = libmount_parse_full(fstab, /* source = */ NULL, MNT_ITER_FORWARD, &table, &iter);
         if (r == -ENOENT)
                 return 0;
         if (r < 0)
@@ -1442,7 +1442,7 @@ static int add_mounts_from_creds(bool prefix_sysroot) {
         _cleanup_(mnt_free_tablep) struct libmnt_table *table = NULL;
         _cleanup_(mnt_free_iterp) struct libmnt_iter *iter = NULL;
 
-        r = libmount_parse_full(cred, f, &table, &iter);
+        r = libmount_parse_full(cred, f, MNT_ITER_FORWARD, &table, &iter);
         if (r < 0)
                 return log_error_errno(r, "Failed to parse credential '%s' (as fstab): %m", cred);
 

src/shared/libmount-util.c

@@ -86,6 +86,7 @@ int dlopen_libmount(void) {
 int libmount_parse_full(
                 const char *path,
                 FILE *source,
+                int direction,
                 struct libmnt_table **ret_table,
                 struct libmnt_iter **ret_iter) {
 
@@ -95,13 +96,14 @@ int libmount_parse_full(
 
         /* Older libmount seems to require this. */
         assert(!source || path);
+        assert(IN_SET(direction, MNT_ITER_FORWARD, MNT_ITER_BACKWARD));
 
         r = dlopen_libmount();
         if (r < 0)
                 return r;
 
         table = sym_mnt_new_table();
-        iter = sym_mnt_new_iter(MNT_ITER_FORWARD);
+        iter = sym_mnt_new_iter(direction);
         if (!table || !iter)
                 return -ENOMEM;
 
@@ -126,7 +128,7 @@ int libmount_parse_fstab(
         struct libmnt_table **ret_table,
         struct libmnt_iter **ret_iter) {
 
-        return libmount_parse_full(fstab_path(), NULL, ret_table, ret_iter);
+        return libmount_parse_full(fstab_path(), NULL, MNT_ITER_FORWARD, ret_table, ret_iter);
 }
 
 int libmount_is_leaf(

src/shared/libmount-util.h

@@ -47,6 +47,7 @@ DEFINE_TRIVIAL_CLEANUP_FUNC_FULL_RENAME(struct libmnt_iter*, sym_mnt_free_iter,
 int libmount_parse_full(
                 const char *path,
                 FILE *source,
+                int direction,
                 struct libmnt_table **ret_table,
                 struct libmnt_iter **ret_iter);
 
@@ -55,14 +56,14 @@ static inline int libmount_parse_mountinfo(
                 struct libmnt_table **ret_table,
                 struct libmnt_iter **ret_iter) {
 
-        return libmount_parse_full("/proc/self/mountinfo", source, ret_table, ret_iter);
+        return libmount_parse_full("/proc/self/mountinfo", source, MNT_ITER_FORWARD, ret_table, ret_iter);
 }
 
 static inline int libmount_parse_with_utab(
                 struct libmnt_table **ret_table,
                 struct libmnt_iter **ret_iter) {
 
-        return libmount_parse_full(NULL, NULL, ret_table, ret_iter);
+        return libmount_parse_full(NULL, NULL, MNT_ITER_FORWARD, ret_table, ret_iter);
 }
 
 int libmount_parse_fstab(struct libmnt_table **ret_table, struct libmnt_iter **ret_iter);

src/shared/mount-util.c

@@ -52,7 +52,7 @@ int umount_recursive_full(const char *prefix, int flags, char **keep) {
                 _cleanup_(mnt_free_iterp) struct libmnt_iter *iter = NULL;
                 bool again = false;
 
-                r = libmount_parse_mountinfo(f, &table, &iter);
+                r = libmount_parse_full("/proc/self/mountinfo", f, MNT_ITER_BACKWARD, &table, &iter);
                 if (r < 0)
                         return log_debug_errno(r, "Failed to parse /proc/self/mountinfo: %m");