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

[Christoph Hellwig <hch@infradead.org>]

>   * 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?