On 29/5/26 12:30, Darrick J. Wong wrote:
On Thu, May 28, 2026 at 12:05:35PM +0800, Anand Jain wrote:
Add a helper to check if the target filesystem supports unique f_fsid
tracking across cloned or snapshot instances.
Certain filesystems like XFS, Btrfs, and F2FS ensure unique f_fsid
identifiers per filesystem instance. However, Ext4 derives its f_fsid
directly from its superblock UUID, which leads to identical f_fsid
values on cloned images until the UUID is manually modified by userspace.
Introduce _require_unique_f_fsid() to allow test cases requiring strict
f_fsid uniqueness to skip gracefully on unsupported filesystems.
Signed-off-by: Anand Jain <[email protected]>
---
common/rc | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/common/rc b/common/rc
index 937f478963b4..5446552aed92 100644
--- a/common/rc
+++ b/common/rc
@@ -6314,6 +6314,27 @@ _require_fanotify_ioerrors()
_notrun "$FSTYP does not support fanotify ioerrors"
}
+# Ext4 derives f_fsid from the superblock UUID, meaning clones share the
+# same f_fsid until their UUIDs diverge. Conversely, XFS, Btrfs,
+# and F2FS ensure f_fsid remains unique per filesystem instance (often by
+# deriving it from the UUID and underlying block device.)
+#
+# Across all filesystems, a UUID collision causes libblkid tools to return
+# non-deterministic device mappings. It is ultimately the responsibility
"device mappings", as in /dev/disk/by-id/$UUID ?
Correct.. I'll make it specific.
+# of the userspace utility or use-case to enforce uniqueness when a clone
+# diverges. For details, see mailing list thread discussions titled:
+# "ext4: derive f_fsid from block device to avoid collisions".
How about providing a direct lore link?
Sure, that will be..
Link:
https://lore.kernel.org/linux-ext4/[email protected]/
instead of the title.
Thanks, Anand
--D
+_require_unique_f_fsid()
+{
+ # Skip the test if the filesystem does not enforce unique f_fsids
+ # natively. Checking this dynamically requires recreating a clone
+ # layout, so we use a static lookup based on FSTYP.
+ if [ "$FSTYP" == "ext4" ]; then
+ _notrun "Target filesystem ($FSTYP) does not guarantee unique f_fsid
on clones."
+ fi
+}
+
+
# Computes a percentage of the available space in a filesystem and
# returns that quantity in MB. The percentage must not contain a percent
# sign ("%").
--
2.43.0
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