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

Darrick J. Wong
2020-08-24T02:43:56Z

On Sat, Aug 22, 2020 at 08:33:19AM +0100, Christoph Hellwig wrote:
> > * 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!!!

Been there, done that. Dave suggested some replacement code (which
corrupted the values), then I modified that into a correct version,
which then made smatch angry because it doesn't like code that does bit
shifts on __be64 values.

> > +/* 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).

Hm, ok. I did not know that. :)

> > {
> > + if (dip->di_version >= 3 &&
> > + (dip->di_flags2 & cpu_to_be64(XFS_DIFLAG2_BIGTIME))) {
>
> Do we want a helper for this condition?

Yes, yes we do. Will add.

> > + 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.

Do we care now that checkpatch has been changed to allow up to 100
columns?

> > + 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?

Yes, good suggestion!

> > @@ -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?

As you and Dave have both pointed out, there are plenty of stupid bugs
still in this. I think I'd like to have at least one EXPERIMENTAL cycle
to make sure I didn't commit anything pathologically stupid in here.

<cough> ext4 34-bit sign extension bug <cough>.

--D