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

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

[Moritz Tanner]

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

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

[Lars Ellenberg]

On Fri, Aug 21, 2026 at 10:54:51AM +0200, Moritz Tanner wrote:
> 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.

The other thaw order is the same defect seen from userspace: while the
bdev freeze is still held, fsfreeze -u returns EINVAL although it did
drop its reference, and the filesystem stays frozen until the block
layer holder thaws it.

In-kernel holders get the spurious error as well, whenever a userspace
freeze is held at the same time: xfs_scrub's xchk_fsthaw() carries the
comment "This should always succeed, we have a kernel freeze", and
f2fs_ioc_gc_range() hands the error to userspace.

Tested-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Reviewed-by: Lars Ellenberg <lars.ellenberg@linbit.com>

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

[Christoph Hellwig]

On Fri, Aug 21, 2026 at 10:54:51AM +0200, Moritz Tanner wrote:
> 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

Can you wire this up in xfstests or blktests?

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

[Christian Brauner]

On 2026-08-21 10:54 +0200, Moritz Tanner wrote:
> 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.

This was contentions when we discussed this back then because you can
also run into situations where userspace thinks it's safe to access the
filesystem because they think the thaw succeeded. But anyway, I think we
can try this change at least.

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

[Christian Brauner]

On Fri, 21 Aug 2026 10:54:51 +0200, Moritz Tanner wrote:
> fs: don't return -EINVAL for successful nested thaw

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] fs: don't return -EINVAL for successful nested thaw
      https://git.kernel.org/vfs/vfs/c/fe967191e585