[PATCH v2 0/3] fix s_uuid and f_fsid consistency for cloned filesystems

[PATCH v2 0/3] fix s_uuid and f_fsid consistency for cloned filesystems

[Anand Jain]

v2:
  Derive statfs::f_fsid only when using new 'nouuid'; naming matches with xfs.
 
v1:
 btrfs:
  https://lore.kernel.org/linux-btrfs/cover.1772095546.git.asj@kernel.org/
 ext4:
  https://lore.kernel.org/linux-ext4/e269a49eed2de23eb9f9bd7f506f0fe47696a023.1772095546.git.asj@kernel.org/

Anand Jain (3):
  btrfs: use on-disk uuid for s_uuid in temp_fsid mounts
  btrfs: derive f_fsid from on-disk fsuuid and dev_t
  ext4: derive f_fsid from block device to avoid collisions

 fs/btrfs/disk-io.c |  3 ++-
 fs/btrfs/fs.h      |  1 +
 fs/btrfs/super.c   | 35 +++++++++++++++++++++++++++++++----
 fs/ext4/ext4.h     |  1 +
 fs/ext4/super.c    | 12 ++++++++++--
 5 files changed, 45 insertions(+), 7 deletions(-)

-- 
2.43.0

[PATCH v2 1/3] btrfs: use on-disk uuid for s_uuid in temp_fsid mounts

[Anand Jain]

When mounting a cloned filesystem with a temporary fsuuid (temp_fsid),
layered modules like overlayfs require a persistent identifier.

While the internal in-memory fs_devices->fsid must remain unique, to
distinguish each instance of the mounted filesystem, let s_uuid carry
the original on-disk UUID.

Signed-off-by: Anand Jain <asj@kernel.org>
---
 fs/btrfs/disk-io.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index c835141ee384..90e0369bf682 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3451,7 +3451,8 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	/* Update the values for the current filesystem. */
 	sb->s_blocksize = sectorsize;
 	sb->s_blocksize_bits = blksize_bits(sectorsize);
-	memcpy(&sb->s_uuid, fs_info->fs_devices->fsid, BTRFS_FSID_SIZE);
+	/* Copy on-disk uuid, even for temp_fsid mounts */
+	memcpy(&sb->s_uuid, fs_info->super_copy->fsid, BTRFS_FSID_SIZE);
 
 	mutex_lock(&fs_info->chunk_mutex);
 	ret = btrfs_read_sys_array(fs_info);
-- 
2.43.0

[PATCH v2 2/3] btrfs: derive f_fsid from on-disk fsuuid and dev_t

[Anand Jain]

f_fsid depends on fs_devices->fsid and subvol root id.

For cloned devices either same as the source or dynamical generated
at mount won't suite because tools like fanotify and ima depends on it.

Switch to a stable derivation using the persistent on-disk fsuuid +
root id + devt of the block device for the single device filesystem.
This is consistent as long as the device remains unchanged/replace
(excludes btrfs device replace secnario for now).

This change is only for the single device configs and is behind the
-o nouuid mount option to keep this change compatible with ABI.

Signed-off-by: Anand Jain <asj@kernel.org>
---
 fs/btrfs/fs.h    |  1 +
 fs/btrfs/super.c | 35 +++++++++++++++++++++++++++++++----
 2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
index a4758d94b32e..6e2a5c2bd03c 100644
--- a/fs/btrfs/fs.h
+++ b/fs/btrfs/fs.h
@@ -270,6 +270,7 @@ enum {
 	BTRFS_MOUNT_IGNOREMETACSUMS		= (1ULL << 31),
 	BTRFS_MOUNT_IGNORESUPERFLAGS		= (1ULL << 32),
 	BTRFS_MOUNT_REF_TRACKER			= (1ULL << 33),
+	BTRFS_MOUNT_NOUUID			= (1ULL << 34),
 };
 
 /* These mount options require a full read-only fs, no new transaction is allowed. */
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 125fca57c164..2fb82032f1e1 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -102,6 +102,7 @@ enum {
 	Opt_compress_type,
 	Opt_degraded,
 	Opt_device,
+	Opt_nouuid,
 	Opt_fatal_errors,
 	Opt_flushoncommit,
 	Opt_max_inline,
@@ -227,6 +228,7 @@ static const struct fs_parameter_spec btrfs_fs_parameters[] = {
 	fsparam_flag_no("datasum", Opt_datasum),
 	fsparam_flag("degraded", Opt_degraded),
 	fsparam_string("device", Opt_device),
+	fsparam_flag("nouuid", Opt_nouuid),
 	fsparam_flag_no("discard", Opt_discard),
 	fsparam_enum("discard", Opt_discard_mode, btrfs_parameter_discard),
 	fsparam_enum("fatal_errors", Opt_fatal_errors, btrfs_parameter_fatal_errors),
@@ -382,6 +384,9 @@ static int btrfs_parse_param(struct fs_context *fc, struct fs_parameter *param)
 			return PTR_ERR(device);
 		break;
 	}
+	case Opt_nouuid:
+		btrfs_set_opt(ctx->mount_opt, NOUUID);
+		break;
 	case Opt_datasum:
 		if (result.negated) {
 			btrfs_set_opt(ctx->mount_opt, NODATASUM);
@@ -1113,6 +1118,8 @@ static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
 		seq_puts(seq, ",discard");
 	if (btrfs_test_opt(info, DISCARD_ASYNC))
 		seq_puts(seq, ",discard=async");
+	if (btrfs_test_opt(info, NOUUID))
+		seq_puts(seq, ",nouuid");
 	if (!(info->sb->s_flags & SB_POSIXACL))
 		seq_puts(seq, ",noacl");
 	if (btrfs_free_space_cache_v1_active(info))
@@ -1733,7 +1740,7 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	u64 total_free_data = 0;
 	u64 total_free_meta = 0;
 	u32 bits = fs_info->sectorsize_bits;
-	__be32 *fsid = (__be32 *)fs_info->fs_devices->fsid;
+	__be32 *fsid;
 	unsigned factor = 1;
 	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
 	int ret;
@@ -1819,15 +1826,35 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_bsize = fs_info->sectorsize;
 	buf->f_namelen = BTRFS_NAME_LEN;
 
-	/* We treat it as constant endianness (it doesn't matter _which_)
-	   because we want the fsid to come out the same whether mounted
-	   on a big-endian or little-endian host */
+	/*
+	 * fs_devices->fsid is dynamically generated when temp_fsid is active
+	 * to support cloned devices. Use the original on-disk fsid instead,
+	 * as it remains consistent across mount cycles.
+	 */
+	fsid = (__be32 *)fs_info->super_copy->fsid;
+	/*
+	 * We treat it as constant endianness (it doesn't matter _which_)
+	 * because we want the fsid to come out the same whether mounted
+	 * on a big-endian or little-endian host.
+	 */
 	buf->f_fsid.val[0] = be32_to_cpu(fsid[0]) ^ be32_to_cpu(fsid[2]);
 	buf->f_fsid.val[1] = be32_to_cpu(fsid[1]) ^ be32_to_cpu(fsid[3]);
 	/* Mask in the root object ID too, to disambiguate subvols */
 	buf->f_fsid.val[0] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root) >> 32;
 	buf->f_fsid.val[1] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root);
 
+	/*
+	 * dev_t provides way to differentiate mounted cloned devices keeps
+	 * the statfs fid is consistent and unique.
+	 */
+	if (btrfs_test_opt(fs_info, NOUUID) &&
+	    fs_info->fs_devices->total_devices == 1) {
+		__kernel_fsid_t dev_fsid = \
+	u64_to_fsid(huge_encode_dev(fs_info->fs_devices->latest_dev->bdev->bd_dev));
+		buf->f_fsid.val[0] ^= dev_fsid.val[1];
+		buf->f_fsid.val[1] ^= dev_fsid.val[0];
+	}
+
 	return 0;
 }
 
-- 
2.43.0

[PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Anand Jain]

statfs() currently reports f_fsid derived from the on-disk UUID.
Cloned block devices share the same UUID, so distinct ext4 instances
can return identical f_fsid values. This leads to collisions in
fanotify.

Encode sb->s_dev into f_fsid instead of using the superblock UUID.
This provides a per-device identifier and avoids conflicts when
filesystem is cloned, matching the behavior with xfs.

Place this change behind the new mount option "-o nouuid" for ABI
compatibility.

Signed-off-by: Anand Jain <asj@kernel.org>
---
 fs/ext4/ext4.h  |  1 +
 fs/ext4/super.c | 12 ++++++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 293f698b7042..64d98ab64c11 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1282,6 +1282,7 @@ struct ext4_inode_info {
 						    * scanning in mballoc
 						    */
 #define EXT4_MOUNT2_ABORT		0x00000100 /* Abort filesystem */
+#define EXT4_MOUNT2_NOUUID		0x00000200 /* No duplicate f_fsid for cloned filesystem */
 
 #define clear_opt(sb, opt)		EXT4_SB(sb)->s_mount_opt &= \
 						~EXT4_MOUNT_##opt
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 43f680c750ae..65b712af4ad7 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1664,7 +1664,7 @@ static const struct export_operations ext4_export_ops = {
 enum {
 	Opt_bsd_df, Opt_minix_df, Opt_grpid, Opt_nogrpid,
 	Opt_resgid, Opt_resuid, Opt_sb,
-	Opt_nouid32, Opt_debug, Opt_removed,
+	Opt_nouid32, Opt_debug, Opt_removed, Opt_nouuid,
 	Opt_user_xattr, Opt_acl,
 	Opt_auto_da_alloc, Opt_noauto_da_alloc, Opt_noload,
 	Opt_commit, Opt_min_batch_time, Opt_max_batch_time, Opt_journal_dev,
@@ -1743,6 +1743,7 @@ static const struct fs_parameter_spec ext4_param_specs[] = {
 	fsparam_u32	("sb",			Opt_sb),
 	fsparam_enum	("errors",		Opt_errors, ext4_param_errors),
 	fsparam_flag	("nouid32",		Opt_nouid32),
+	fsparam_flag	("nouuid",		Opt_nouuid),
 	fsparam_flag	("debug",		Opt_debug),
 	fsparam_flag	("oldalloc",		Opt_removed),
 	fsparam_flag	("orlov",		Opt_removed),
@@ -1898,6 +1899,7 @@ static const struct mount_opts {
 	{Opt_acl, 0, MOPT_NOSUPPORT},
 #endif
 	{Opt_nouid32, EXT4_MOUNT_NO_UID32, MOPT_SET},
+	{Opt_nouuid, EXT4_MOUNT2_NOUUID, MOPT_SET | MOPT_2},
 	{Opt_debug, EXT4_MOUNT_DEBUG, MOPT_SET},
 	{Opt_quota, EXT4_MOUNT_QUOTA | EXT4_MOUNT_USRQUOTA, MOPT_SET | MOPT_Q},
 	{Opt_usrquota, EXT4_MOUNT_QUOTA | EXT4_MOUNT_USRQUOTA,
@@ -2389,6 +2391,9 @@ static int ext4_parse_param(struct fs_context *fc, struct fs_parameter *param)
 			return -EINVAL;
 		}
 		return 0;
+	case Opt_nouuid:
+		ctx_set_mount_opt2(ctx, EXT4_MOUNT2_NOUUID);
+		return 0;
 	}
 
 	/*
@@ -6941,7 +6946,10 @@ static int ext4_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_files = le32_to_cpu(es->s_inodes_count);
 	buf->f_ffree = percpu_counter_sum_positive(&sbi->s_freeinodes_counter);
 	buf->f_namelen = EXT4_NAME_LEN;
-	buf->f_fsid = uuid_to_fsid(es->s_uuid);
+	if (test_opt2(sb, NOUUID))
+		buf->f_fsid = u64_to_fsid(huge_encode_dev(sb->s_bdev->bd_dev));
+	else
+		buf->f_fsid = uuid_to_fsid(es->s_uuid);
 
 #ifdef CONFIG_QUOTA
 	if (ext4_test_inode_flag(dentry->d_inode, EXT4_INODE_PROJINHERIT) &&
-- 
2.43.0

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Theodore Tso]

On Sat, Mar 21, 2026 at 07:55:19PM +0800, Anand Jain wrote:
> statfs() currently reports f_fsid derived from the on-disk UUID.
> Cloned block devices share the same UUID, so distinct ext4 instances
> can return identical f_fsid values. This leads to collisions in
> fanotify.
> 
> Encode sb->s_dev into f_fsid instead of using the superblock UUID.
> This provides a per-device identifier and avoids conflicts when
> filesystem is cloned, matching the behavior with xfs.

As I observed in [1] this leads to collisions when for removable block
devices which can be used to mount different file systems.

[1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/

> Place this change behind the new mount option "-o nouuid" for ABI
> compatibility.

I *really* hate this mount option.  It's not at all obvious what it
means for a system administrator who hasn't had the context of reading
the e-mail discussion on this subject.

As I stated in [1], I think the f_fsid is a terrible interface that
was promulgated by history, and future usage should be strongly
discouraged, and the wise programmer won't use it because it has
significant compatibility issues.

As such, my personal preference is that we not try to condition it on
a mount option, which in all likelihood almost no one will use, and
instead just change it so that we hash the file system's UUID and
block device number together and use that for ext4's f_fsid.

Thoughts, comments?

						- Ted

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Darrick J. Wong]

On Sun, Mar 22, 2026 at 11:16:24PM -0500, Theodore Tso wrote:
> On Sat, Mar 21, 2026 at 07:55:19PM +0800, Anand Jain wrote:
> > statfs() currently reports f_fsid derived from the on-disk UUID.
> > Cloned block devices share the same UUID, so distinct ext4 instances
> > can return identical f_fsid values. This leads to collisions in
> > fanotify.
> > 
> > Encode sb->s_dev into f_fsid instead of using the superblock UUID.
> > This provides a per-device identifier and avoids conflicts when
> > filesystem is cloned, matching the behavior with xfs.
> 
> As I observed in [1] this leads to collisions when for removable block
> devices which can be used to mount different file systems.
> 
> [1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/
> 
> > Place this change behind the new mount option "-o nouuid" for ABI
> > compatibility.
> 
> I *really* hate this mount option.  It's not at all obvious what it
> means for a system administrator who hasn't had the context of reading
> the e-mail discussion on this subject.

I don't love 'nouuid' either, because it means something completely
different in XFS.  'fsid_from_dev' or something would at least be
clearer about what it's doing...

> As I stated in [1], I think the f_fsid is a terrible interface that
> was promulgated by history, and future usage should be strongly
> discouraged, and the wise programmer won't use it because it has
> significant compatibility issues.
> 
> As such, my personal preference is that we not try to condition it on
> a mount option, which in all likelihood almost no one will use, and
> instead just change it so that we hash the file system's UUID and
> block device number together and use that for ext4's f_fsid.

...but why not just set fsid to some approximation of the dev_t like
XFS and be done with it?

	st->f_fsid = u64_to_fsid(huge_encode_dev(mp->m_ddev_targp->bt_dev))

There are a few other single-bdev filesystems that do this.

--D

> Thoughts, comments?
> 
> 						- Ted
> 

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Darrick J. Wong]

On Mon, Mar 23, 2026 at 08:29:43AM -0700, Darrick J. Wong wrote:
> On Sun, Mar 22, 2026 at 11:16:24PM -0500, Theodore Tso wrote:
> > On Sat, Mar 21, 2026 at 07:55:19PM +0800, Anand Jain wrote:
> > > statfs() currently reports f_fsid derived from the on-disk UUID.
> > > Cloned block devices share the same UUID, so distinct ext4 instances
> > > can return identical f_fsid values. This leads to collisions in
> > > fanotify.
> > > 
> > > Encode sb->s_dev into f_fsid instead of using the superblock UUID.
> > > This provides a per-device identifier and avoids conflicts when
> > > filesystem is cloned, matching the behavior with xfs.
> > 
> > As I observed in [1] this leads to collisions when for removable block
> > devices which can be used to mount different file systems.
> > 
> > [1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/
> > 
> > > Place this change behind the new mount option "-o nouuid" for ABI
> > > compatibility.
> > 
> > I *really* hate this mount option.  It's not at all obvious what it
> > means for a system administrator who hasn't had the context of reading
> > the e-mail discussion on this subject.
> 
> I don't love 'nouuid' either, because it means something completely
> different in XFS.  'fsid_from_dev' or something would at least be
> clearer about what it's doing...
> 
> > As I stated in [1], I think the f_fsid is a terrible interface that
> > was promulgated by history, and future usage should be strongly
> > discouraged, and the wise programmer won't use it because it has
> > significant compatibility issues.
> > 
> > As such, my personal preference is that we not try to condition it on
> > a mount option, which in all likelihood almost no one will use, and
> > instead just change it so that we hash the file system's UUID and
> > block device number together and use that for ext4's f_fsid.
> 
> ...but why not just set fsid to some approximation of the dev_t like
> XFS and be done with it?
> 
> 	st->f_fsid = u64_to_fsid(huge_encode_dev(mp->m_ddev_targp->bt_dev))
> 
> There are a few other single-bdev filesystems that do this.

/me reads the rest of both threads, realizes the answer is "because you
can eject media, mount another media with the same fs, and bizarrely
they produce the same fsid."

I'll point out that for the filesystems that base fsid off of a block
device, perhaps we should mix in the diskseq to prevent that problem?

But then someone said the magic words "IMA and fanotify also use fsid"
and /me backs away from yet another opportunity to argue about
open-coded file handles from legacy Unix.

--D

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Andreas Dilger]

On Mar 23, 2026, at 09:29, Darrick J. Wong <djwong@kernel.org> wrote:
> 
> On Sun, Mar 22, 2026 at 11:16:24PM -0500, Theodore Tso wrote:
>> On Sat, Mar 21, 2026 at 07:55:19PM +0800, Anand Jain wrote:
>>> statfs() currently reports f_fsid derived from the on-disk UUID.
>>> Cloned block devices share the same UUID, so distinct ext4 instances
>>> can return identical f_fsid values. This leads to collisions in
>>> fanotify.
>>> 
>>> Encode sb->s_dev into f_fsid instead of using the superblock UUID.
>>> This provides a per-device identifier and avoids conflicts when
>>> filesystem is cloned, matching the behavior with xfs.
>> 
>> As I observed in [1] this leads to collisions when for removable block
>> devices which can be used to mount different file systems.
>> 
>> [1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/
>> 
>>> Place this change behind the new mount option "-o nouuid" for ABI
>>> compatibility.
>> 
>> I *really* hate this mount option.  It's not at all obvious what it
>> means for a system administrator who hasn't had the context of reading
>> the e-mail discussion on this subject.
> 
> I don't love 'nouuid' either, because it means something completely
> different in XFS.  'fsid_from_dev' or something would at least be
> clearer about what it's doing...
> 
>> As I stated in [1], I think the f_fsid is a terrible interface that
>> was promulgated by history, and future usage should be strongly
>> discouraged, and the wise programmer won't use it because it has
>> significant compatibility issues.
>> 
>> As such, my personal preference is that we not try to condition it on
>> a mount option, which in all likelihood almost no one will use, and
>> instead just change it so that we hash the file system's UUID and
>> block device number together and use that for ext4's f_fsid.
> 
> ...but why not just set fsid to some approximation of the dev_t like
> XFS and be done with it?
> 
> st->f_fsid = u64_to_fsid(huge_encode_dev(mp->m_ddev_targp->bt_dev))
> 
> There are a few other single-bdev filesystems that do this.

On the flip side, if the fsid of a filesystem changes because a new disk
was installed on a server and the old disk gets a new device number or an
upgraded kernel with different device driver load ordering, that would
also be a big problem, as it would break NFS file handles over a reboot.

The whole point of generating the fsid from the persistent storage is that
it is persistent across reboots.  It seems like the real issue here is
cloning filesystem images and not assigning a new UUID to the cloned image.

Cheers, Andreas

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Anand Jain]

On 25/3/26 18:02, Andreas Dilger wrote:
> On Mar 23, 2026, at 09:29, Darrick J. Wong <djwong@kernel.org> wrote:
>>
>> On Sun, Mar 22, 2026 at 11:16:24PM -0500, Theodore Tso wrote:
>>> On Sat, Mar 21, 2026 at 07:55:19PM +0800, Anand Jain wrote:
>>>> statfs() currently reports f_fsid derived from the on-disk UUID.
>>>> Cloned block devices share the same UUID, so distinct ext4 instances
>>>> can return identical f_fsid values. This leads to collisions in
>>>> fanotify.
>>>>
>>>> Encode sb->s_dev into f_fsid instead of using the superblock UUID.
>>>> This provides a per-device identifier and avoids conflicts when
>>>> filesystem is cloned, matching the behavior with xfs.
>>>
>>> As I observed in [1] this leads to collisions when for removable block
>>> devices which can be used to mount different file systems.
>>>
>>> [1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/
>>>
>>>> Place this change behind the new mount option "-o nouuid" for ABI
>>>> compatibility.
>>>
>>> I *really* hate this mount option.  It's not at all obvious what it
>>> means for a system administrator who hasn't had the context of reading
>>> the e-mail discussion on this subject.
>>
>> I don't love 'nouuid' either, because it means something completely
>> different in XFS.  'fsid_from_dev' or something would at least be
>> clearer about what it's doing...
>>
>>> As I stated in [1], I think the f_fsid is a terrible interface that
>>> was promulgated by history, and future usage should be strongly
>>> discouraged, and the wise programmer won't use it because it has
>>> significant compatibility issues.
>>>
>>> As such, my personal preference is that we not try to condition it on
>>> a mount option, which in all likelihood almost no one will use, and
>>> instead just change it so that we hash the file system's UUID and
>>> block device number together and use that for ext4's f_fsid.
>>
>> ...but why not just set fsid to some approximation of the dev_t like
>> XFS and be done with it?
>>
>> st->f_fsid = u64_to_fsid(huge_encode_dev(mp->m_ddev_targp->bt_dev))
>>
>> There are a few other single-bdev filesystems that do this.
> 
> On the flip side, if the fsid of a filesystem changes because a new disk
> was installed on a server and the old disk gets a new device number or an
> upgraded kernel with different device driver load ordering, that would
> also be a big problem, as it would break NFS file handles over a reboot.
> 
> The whole point of generating the fsid from the persistent storage is that
> it is persistent across reboots.  It seems like the real issue here is
> cloning filesystem images and not assigning a new UUID to the cloned image.
> 

IMO, sb->s_uuid (as used by overlayfs)
Represents a filesystem UUID that is persistent.
It is derived from on-disk metadata.

statfs()->f_fsid is..
A kind of runtime filesystem identifier used to distinguish mounted
filesystems within a running system.
It may be stable across reboots or device removal and reinsertion,
but this is not guaranteed. It may change if the device dev_t changes.

I have posted a set of five test cases to the mailing list
to help verify these behaviors, for your review. Another
test case to verify device reinsertion with a different
dev_t is a WIP; it will be submitted along with v3.

Thanks.

> Cheers, Andreas

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Theodore Tso]

On Wed, Mar 25, 2026 at 06:59:32PM +0800, Anand Jain wrote:
> 
> IMO, sb->s_uuid (as used by overlayfs)
> Represents a filesystem UUID that is persistent.
> It is derived from on-disk metadata.
> 
> statfs()->f_fsid is..
> A kind of runtime filesystem identifier used to distinguish mounted
> filesystems within a running system.
> It may be stable across reboots or device removal and reinsertion,
> but this is not guaranteed. It may change if the device dev_t changes.

I always worry about "it might be stable, but it might not; ¯\_(ツ)_/¯"

The problem with that is that people might starting using this
kinda-of-guarantee-but-maybe-not in scripts or in programs, and then
when people try to run that script or program on a different system,
or on a different file system, things goes *boom*.

So if we want to say that it is stable so long as dev_t and the file
system the same, that's a well defined semantic.

If it's that it has no guarantees whatsoever; cloud change across
reboots; could change across remounts, then maybe it should just be a
global mount sequence number that starts with a random number at boot.
So you can use it to distinguish between different mounted file
systems, but that's *all* you can do with the thing.  That would also
be a well defined semantic.

Cheers,

					- Ted

Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Anand Jain]

Thanks for the feedback. I'll try to address the points raised
here and in your earlier email [1].

  [1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/

This work originally came out of a Btrfs issue where a cloned
filesystem ended up using a dynamically generated, mount-time
UUID for sb->s_uuid instead of the on-disk UUID. As a result,
OverlayFS (with index enabled) started failing mount-recycle
tests [2] for the cloned filesystem.


 [2]
https://lore.kernel.org/lkml/20251014015707.129013-1-andrealmeid@igalia.com/

While looking into that problem, I also noticed that different
filesystems derive f_fsid in inconsistent ways, and in practice
many of them base it on dev_t.

On the question of the 64-bit limit: although a 64-bit value
is not globally unique in the way a 128-bit UUID is, f_fsid
has historically been derived from dev_t. Since dev_t must be
unique within a running kernel instance, 64 bits are enough to
safely encode its effective ~32-bit dev_t without collisions.
The number of concurrently addressable block devices is also
bounded by the 12-bit major / 20-bit minor limits and
/proc/sys/fs/mount-max. IMO, within a single boot, 64 bits
should provide a collision-free identifier.

I've also submitted new test cases that validate expectations
around both sb->s_uuid and statfs::f_fsid here [3].

  [3] https://lore.kernel.org/fstests/cover.1774090817.git.asj@kernel.org/

> As I observed in [1] this leads to collisions when for removable block
> devices which can be used to mount different file systems.
>
> [1] https://lore.kernel.org/all/20260322203151.GA98947@mac.lan/

I agree. A straightforward f_fsid = f(dev_t) will collide if a
removable device is swapped but ends up reusing the same dev_t.
Theoretically, I see this can be reproduced with XFS, and with
my current patchset on Ext4. That’s clearly a blocker, and I plan
to revise, btw Btrfs does well for this test scenario.

> And even as you've proposed to change
> things, it's not consistent across file systems.  In particular, your
> proposed solution mixes s_uuid into btrfs-patched, but not
> ext4-patched.  Why?

The discrepancy exists because Btrfs must distinguish subvolume
mounts as separate logical entities. For Btrfs, the derivation
requires f(s_uuid, root_id, dev_t) to ensure that two different
subvolumes on the same device report distinct f_fsid values.
For Ext4, a simpler f(s_uuid, dev_t) should suffice to ensure
both cross-device uniqueness and persistent across media swaps.


>> Place this change behind the new mount option "-o nouuid" for ABI
>> compatibility.
> 
> I *really* hate this mount option.  It's not at all obvious what it
> means for a system administrator who hasn't had the context of reading
> the e-mail discussion on this subject.
> 
> As I stated in [1], I think the f_fsid is a terrible interface that
> was promulgated by history, and future usage should be strongly
> discouraged, and the wise programmer won't use it because it has
> significant compatibility issues.
> 
> As such, my personal preference is that we not try to condition it on
> a mount option, which in all likelihood almost no one will use, and
> instead just change it so that we hash the file system's UUID and
> block device number together and use that for ext4's f_fsid.

The decision to gate this behind a mount option followed feedback
from Christoph Hellwig. The concern is binary compatibility:
applications that manually derive an ID based on existing behavior
might break if the kernel changes its derivation logic.

I agree that -o nouuid is a poor name. If we must keep the mount
option  for ABI stability, I am open to better nomenclature.

If we agree that f_fsid is already a problematic interface and
should simply be fixed without any special options for example by
always hashing the filesystem UUID together with the block device
number for Ext4, that would be my preference.

Thanks, Anand