[PATCH] fs: don't return -EINVAL for successful nested thaw

[Moritz Tanner <moritz.tanner@linbit.com>]

Commit 7366f8b6fc6a ("fs: handle freezing from multiple devices")
replaced the freeze_holders bitmask with per-holder counters to allow
nested freezes. In the bitmask version, a thaw that released a shared
hold while another holder remained returned 0. Since the rework,
thaw_super_locked() drops the freeze reference via freeze_dec() but
then returns -EINVAL when other freezers remain, misinforming the
caller: the thaw did succeed, the superblock just stays frozen for the
remaining holders.

This breaks bdev-initiated freezing. When a filesystem is frozen with
FIFREEZE and additionally frozen via bdev_freeze() -- which nests by
design, see fs_bdev_freeze() -- the subsequent bdev_thaw() receives
-EINVAL from the holder op although its freeze reference was dropped,
and therefore keeps bd_fsfreeze_count elevated. Then device-mapper's
unlock_fs() ignores bdev_thaw()'s return value, so nothing rebalances
the count. After the user's FITHAW and umount, the block device can
never be mounted again:

    dm-1: Can't mount, blockdev is frozen

There is no way for userspace to drop the leaked count; only
destroying the block device (or a reboot) recovers the device.

Reproducer (any kernel since v6.8):

    dmsetup create dut --table "0 $(blockdev --getsz "$DEV") linear $DEV 0"
    mkfs.ext4 /dev/mapper/dut
    mount /dev/mapper/dut /mnt
    fsfreeze --freeze /mnt      # freeze_ucount == 1
    dmsetup suspend dut         # bd_fsfreeze_count == 1, ucount == 2
    dmsetup resume dut          # ucount 2 -> 1, but thaw_super()
                                # returns -EINVAL, so bdev_thaw()
                                # keeps bd_fsfreeze_count at 1
    fsfreeze --unfreeze /mnt    # filesystem thaws fine
    umount /mnt
    mount /dev/mapper/dut /mnt  # EBUSY, forever

The same happens with fsfreeze held across an LVM snapshot of the
origin volume.

fs_bdev_thaw()'s documentation already describes the intended
semantics: "If this function returns zero it doesn't mean that the
filesystem is unfrozen as it may have been frozen multiple times".
Restore them by returning 0 when a nested thaw drops its hold while
other freezers remain. Thawing without holding a freeze still fails
with -EINVAL as may_unfreeze() rejects that case before the reference
count is touched.

Fixes: 7366f8b6fc6a ("fs: handle freezing from multiple devices")
Cc: <stable@vger.kernel.org> # needs adjustments for < 6.17 (no may_unfreeze())
Signed-off-by: Moritz Tanner <moritz.tanner@linbit.com>
---
 fs/super.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/super.c b/fs/super.c
index 05e443173038..01db6124e409 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -2369,11 +2369,14 @@ static int thaw_super_locked(struct super_block *sb, enum freeze_holder who,
 		goto out_unlock;
 
 	/*
-	 * All freezers share a single active reference.
-	 * So just unlock in case there are any left.
+	 * All freezers share a single active reference. If other freezers
+	 * remain, drop our hold and report success; the superblock stays
+	 * frozen until the last holder thaws it.
 	 */
-	if (freeze_dec(sb, who))
+	if (freeze_dec(sb, who)) {
+		error = 0;
 		goto out_unlock;
+	}
 
 	if (sb_rdonly(sb)) {
 		sb->s_writers.frozen = SB_UNFROZEN;
-- 
2.55.0