zfs_ctldir: make .zfs/snapshot/<name> btime the snapshot creation time

[ixhamza]

Motivation and Context

.zfs/snapshot/<name> reports a different birth time depending on whether the snapshot is automounted. Before the automount it is a placeholder inode with the kernel's simple_dir_inode_operations, which has no getattr, so statx(2) returns no stx_btime at all (a/m/ctime are the snapshot creation time since a21ca18). Once mounted it is the snapshot's root directory and reports that directory's crtime, i.e. the dataset creation time. So a consumer that wants "when was this snapshot taken" from a plain statx(2) gets nothing or the dataset creation time, and the answer changes when the snapshot gets mounted. Reporting the snapshot creation time as stx_btime in both states gives one consistent value for the path; birth time is the natural field for it and the one least likely to affect existing consumers.

Description

The unmounted entry gets its own inode_operations table, zpl_ops_snapdirs, so it has a getattr at all. It reports the creation time that zfsctl_inode_alloc() already looked up for a/m/ctime, now also kept in z_btime. For the mounted case, zfs_getattr_fast() already fakes the inode number of a snapshot root so NFS sees the same fileid before and after the automount; the birth time is now faked in the same place, from dsl_get_creation() of the mounted dataset. To make that stick, the STATX_BTIME copy in zpl_getattr() had to move down into zfs_getattr_fast(), otherwise it would overwrite the override on the way out.

How Has This Been Tested?

Before the patch:

# zfs get -H -o value creation tank/ds@snap1
Wed Sep  9  8:28 2026
# P=/mnt/tank/ds/.zfs/snapshot/snap1

# before mounting (stat does not trigger the automount): no birth time
# stat -c 'birth=%w' $P
birth=-

# after mounting (ls into the directory triggers the automount):
# the root directory's crtime, i.e. the dataset creation time
# ls $P >/dev/null; stat -c 'birth=%w' $P
birth=2026-09-09 08:28:57.959575818 -0700

After the patch:

# before mounting: the snapshot creation time
# stat -c 'birth=%w' $P
birth=2026-09-09 08:28:58.000000000 -0700

# after mounting: same
# ls $P >/dev/null; stat -c 'birth=%w' $P
birth=2026-09-09 08:28:58.000000000 -0700

• Also verified that .zfs, .zfs/snapshot and files inside the snapshot are unaffected, and that the value holds across rename, rollback, promote, send/recv and expiry unmount, including over NFSv4 and SMB.

• ZTS: snapdir_stat_btime fails on an unpatched module at the first check and passes with the change; the snapdir, snapshot, crtime, stat and ctime groups pass. Also compiled against 6.1 headers.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.