Hi Karel,
I noticed that "umount -r" does not work on my system for filesystems other than root:
# umount -r /usr
umount: /usr: target is busy.
Yes, umount first tries the umount2 syscall, so "target is busy" is very expected, but there is no follow-up attempt to remount fs as read-only:
umount2("/usr", 0) = -1 EBUSY (Device or resource busy)
openat(AT_FDCWD, "/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=2998, ...}, AT_EMPTY_PATH) = 0
read(3, "# Locale name alias data base.\n#"..., 4096) = 2998
read(3, "", 4096) = 0
close(3) = 0
openat(AT_FDCWD, "/usr/share/locale/en_US/LC_MESSAGES/util-linux.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/usr/share/locale/en/LC_MESSAGES/util-linux.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
write(2, "umount: ", 8umount: ) = 8
write(2, "/usr: target is busy.", 21/usr: target is busy.) = 21
write(2, "\n", 1
) = 1
dup(1) = 3
close(3) = 0
dup(2) = 3
close(3) = 0
exit_group(32) = ?
+++ exited with 32 +++
Looking at the code, "try remount read-only" section in do_umount() from libmount/src/context_umount.c seems to only be called if all these conditions are met:
- rc < 0
- cxt->syscall_status == -EBUSY
- mnt_context_is_rdonly_umount(cxt)
- src is not NULL
I added some debug code to do_umount() to see why it fails:
DBG(CXT, ul_debugobj(cxt, "rc=%d, cxt->syscall_status=%d, mnt_context_is_rdonly_umount=%d, src=%s",
rc, cxt->syscall_status, mnt_context_is_rdonly_umount(cxt), src));
Result:
17555: libmount: CXT: [0x5638caf185b0]: rc=-1, cxt->syscall_status=-16, mnt_context_is_rdonly_umount=1, src=(null)
With this, I noticed the additional hint in the log coming from lookup_umount_fs_by_statfs():
17555: libmount: CXT: [0x5638caf185b0]: umount: lookup FS
17555: libmount: CXT: [0x5638caf185b0]: lookup by statfs
17555: libmount: CXT: [0x5638caf185b0]: trying fstatfs()
17555: libmount: CXT: [0x5638caf185b0]: umount: disabling mountinfo
Adding "DBG(CXT, mnt_fs_print_debug(cxt->fs, stderr));" in do_umount() confirmed my assumptions:
19114: libmount: CXT: ------ fs:
source: (null)
target: /usr
fstype: ext4
The problem seems to be that lookup_umount_fs() first calls lookup_umount_fs_by_statfs() and when it succeeds, we only get partial information - without the source.
Commenting the following section in lookup_umount_fs():
// rc = lookup_umount_fs_by_statfs(cxt, tgt);
// if (rc <= 0)
// goto done;
... allows this to work and /usr gets re-mounted ro:
23821: libmount: UPDATE: ------ fs:
source: /dev/mapper/VG0-usr
target: /usr
fstype: ext3
optstr: rw,defaults,data=journal,nodev,remount
VFS-optstr: rw,nodev,remount
FS-opstr: data=journal
user-optstr: defaults
umount2("/usr", 0) = -1 EBUSY (Device or resource busy)
mount("/dev/mapper/VG0-usr", "/usr", NULL, MS_RDONLY|MS_REMOUNT, NULL) = 0
Clearly this is not the right fix, but perhaps something like this would be correct:
@@ -275,6 +275,7 @@
|| mnt_context_is_lazy(cxt)
|| mnt_context_is_nocanonicalize(cxt)
|| mnt_context_is_loopdel(cxt)
+ || mnt_context_is_rdonly_umount(cxt)
|| mnt_safe_stat(tgt, &st) != 0 || !S_ISDIR(st.st_mode)
|| has_utab_entry(cxt, tgt))
return 1; /* not found */
I wonder if we just missed the mnt_context_is_rdonly_umount case in https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/?id=6a52473ecd877227f6f7da2b95da0b51593ffec1?
Thanks,
Krzysztof
