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btrfs: derive f_fsid with dev_t only when temp_fsid is active

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Commit c2a74ed049 ("btrfs: derive f_fsid from on-disk fsid and dev_t")
mixed dev_t into f_fsid for all single-device setups to avoid f_fsid
collisions with cloned filesystems.

However, doing this unconditionally breaks backward compatibility.
statfs(2) f_fsid changes after a kernel upgrade, and also can shift
across reboots or dev re-attaches as dev_t values change.

Fix this by only mixing dev_t when temp_fsid is active.  This means for
non-temp_fsid setups or the original mount, we use the old method of
deriving fsid based on the UUID.

So in the case of a cloned Btrfs filesystem, we won't be able to
maintain the same fsid across mount recycle if the mount order changes.

Reported-by: Dave Hansen <dave.hansen@intel.com>
Link: https://lore.kernel.org/linux-btrfs/be0c08f5-2f31-40f5-8a3b-f2f58b3e00ff@intel.com
Fixes: c2a74ed049 ("btrfs: derive f_fsid from on-disk fsid and dev_t")
CC: stable@vger.kernel.org # 7.2
Signed-off-by: Anand Jain <asj@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Anand Jain 2026-09-13 02:06:28 +08:00 committed by David Sterba
parent b797b52e88
commit 72de4807ba
1 changed files with 6 additions and 2 deletions
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8
fs/btrfs/super.c
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@ -1836,8 +1836,12 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
f_fsid.val[0] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root) >> 32;
f_fsid.val[1] ^= btrfs_root_id(BTRFS_I(d_inode(dentry))->root);
/* Hash dev_t to avoid f_fsid collision with cloned filesystems. */
if (fs_info->fs_devices->total_devices == 1) {
/*
* Hash dev_t to avoid f_fsid collisions with cloned filesystems.
* Only do this when a clone is present so the original filesystem
* (mounted first) maintains backward-compatible f_fsid behavior.
*/
if (fs_info->fs_devices->temp_fsid) {
__kernel_fsid_t dev_fsid =
u64_to_fsid(huge_encode_dev(fs_info->fs_devices->latest_dev->bdev->bd_dev));