jbd2: avoid false sharing with j_state_lock

is_journal_aborted() reads journal->j_flags from ext4 and JBD2 hot
paths. At the same time, transaction start and stop paths repeatedly
acquire and release j_state_lock. read_lock() and read_unlock() update
the lock word, so this traffic invalidates the cache line containing
j_flags.

On the tested x86-64 build, j_flags, j_errno, and j_state_lock occupy
the same 64-byte cache line. j_flags starts at offset 0, j_errno at
offset 8, and j_state_lock at offset 56.

Align j_state_lock to a cacheline boundary on SMP builds. This moves it
to offset 64 and prevents lock traffic from invalidating j_flags and
j_errno. Tests were run on Linux 7.3-rc1.

On a two-socket Intel Xeon Silver 4208 system with SMT disabled, CPUs
fixed at 2 GHz, and 16 stress-ng workers, ten-run average bogo-ops/s for
stress-ng.utime, stress-ng.access, stress-ng.file-ioctl, and
stress-ng.open increased by 9.60%, 5.42%, 2.05%, and 4.65%,
respectively. In separate five-run perf c2c measurements, average
Remote HITM samples for the affected j_flags/j_errno line decreased
from 1996, 2041, 1555, and 383 to zero, respectively.

Signed-off-by: JonasZhou-oc <jonaszhou-oc@zhaoxin.com>
Link: https://patch.msgid.link/20260902070331.820322-1-jonaszhou-oc@zhaoxin.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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