Re: [PATCH 08/11] xfs: widen ondisk timestamps to deal with y2038 problem

Christoph Hellwig
2020-08-22T07:33:24Z

> * in the AGI header so that we can skip the finobt walk at mount time when
> @@ -855,12 +862,18 @@ struct xfs_agfl {
> *
> * Inode timestamps consist of signed 32-bit counters for seconds and
> * nanoseconds; time zero is the Unix epoch, Jan 1 00:00:00 UTC 1970.
> + *
> + * When bigtime is enabled, timestamps become an unsigned 64-bit nanoseconds
> + * counter. Time zero is the start of the classic timestamp range.
> */
> union xfs_timestamp {
> struct {
> __be32 t_sec; /* timestamp seconds */
> __be32 t_nsec; /* timestamp nanoseconds */
> };
> +
> + /* Nanoseconds since the bigtime epoch. */
> + __be64 t_bigtime;
> };

So do we really need the union here? What about:

(1) keep the typedef instead of removing it
(2) switch the typedef to be just a __be64, and use trivial helpers
to extract the two separate legacy sec/nsec field
(3) PROFIT!!!

> +/* Convert an ondisk timestamp into the 64-bit safe incore format. */
> void
> xfs_inode_from_disk_timestamp(
> + struct xfs_dinode *dip,
> struct timespec64 *tv,
> const union xfs_timestamp *ts)

I think passing ts by value might lead to somewhat better code
generation on modern ABIs (and older ABIs just fall back to pass
by reference transparently).

> {
> + if (dip->di_version >= 3 &&
> + (dip->di_flags2 & cpu_to_be64(XFS_DIFLAG2_BIGTIME))) {

Do we want a helper for this condition?

> + uint64_t t = be64_to_cpu(ts->t_bigtime);
> + uint64_t s;
> + uint32_t n;
> +
> + s = div_u64_rem(t, NSEC_PER_SEC, &n);
> + tv->tv_sec = s - XFS_INO_BIGTIME_EPOCH;
> + tv->tv_nsec = n;
> + return;
> + }
> +
> tv->tv_sec = (int)be32_to_cpu(ts->t_sec);
> tv->tv_nsec = (int)be32_to_cpu(ts->t_nsec);

Nit: for these kinds of symmetric conditions and if/else feels a little
more natural.

> + xfs_log_dinode_to_disk_ts(from, &to->di_crtime, &from->di_crtime);

This adds a > 80 char line.

> + if (from->di_flags2 & XFS_DIFLAG2_BIGTIME) {
> + uint64_t t;
> +
> + t = (uint64_t)(ts->tv_sec + XFS_INO_BIGTIME_EPOCH);
> + t *= NSEC_PER_SEC;
> + its->t_bigtime = t + ts->tv_nsec;

This calculation is dupliated in two places, might be worth
adding a little helper (which will need to get the sec/nsec values
passed separately due to the different structures).

> + xfs_inode_to_log_dinode_ts(from, &to->di_crtime, &from->di_crtime);

Another line over 8 characters here.

> + if (xfs_sb_version_hasbigtime(&mp->m_sb)) {
> + sb->s_time_min = XFS_INO_BIGTIME_MIN;
> + sb->s_time_max = XFS_INO_BIGTIME_MAX;
> + } else {
> + sb->s_time_min = XFS_INO_TIME_MIN;
> + sb->s_time_max = XFS_INO_TIME_MAX;
> + }

This is really a comment on the earlier patch, but maybe we should
name the old constants with "OLD" or "LEGACY" or "SMALL" in the name?

> @@ -1494,6 +1499,10 @@ xfs_fc_fill_super(
> if (XFS_SB_VERSION_NUM(&mp->m_sb) == XFS_SB_VERSION_5)
> sb->s_flags |= SB_I_VERSION;
>
> + if (xfs_sb_version_hasbigtime(&mp->m_sb))
> + xfs_warn(mp,
> + "EXPERIMENTAL big timestamp feature in use. Use at your own risk!");
> +

Is there any good reason to mark this experimental?