Re: [PATCH v2 00/11] xfs: widen timestamps to deal with y2038

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Aug 19, 2020 at 09:01:21AM +1000, Dave Chinner wrote:
> On Mon, Aug 17, 2020 at 03:56:48PM -0700, Darrick J. Wong wrote:
> > Hi all,
> > 
> > This series performs some refactoring of our timestamp and inode
> > encoding functions, then retrofits the timestamp union to handle
> > timestamps as a 64-bit nanosecond counter.  Next, it adds bit shifting
> > to the non-root dquot timer fields to boost their effective size to 34
> > bits.  These two changes enable correct time handling on XFS through the
> > year 2486.
> 
> A bit more detail would be nice :)

Heh, ok.

> Like, the inode timestamp has a range of slightly greater than 2^34
> because 10^9 < 2^30. i.e.
> 
> Inode timestamp range in days:
> 
> $ echo $(((2**62 / (1000*1000*1000) / 86400) * 2**2))
> 213500
> $
> 
> While the quota timer range in days is:
> $ echo $(((2**34 / 86400)))
> 198841
> $
> 
> There's ~15,000 days difference in range here, which in years is
> about 40 years. Hence the inodes have a timestamp range out to
> ~2485 from the 1901 epoch start, while quota timers have a range
> out to only 2445 from the epoch start.

Quota timers have always treated the d_{b,i,rtb}timer value as an
unsigned 32-bit integer, which means that it has /never/ been possible
to set a timer expiration before 1/1/1970.  The quota timer range is
therefore 198,841 days *after* 1970, not after 1901.

Therefore, the quota timer range in days is:

$ echo $(( ((2**34) + (2**31)) / 86400) ))
223696

So, technically speaking, the quota timers could go beyond 2486, but the
current patchset clamps the quota counters to the same max as the
inodes.  I guess I just proved the need for more details upfront.

--D

> 
> Some discussion of the different ranges, the problems it might cause
> and why we don't have to worry about it would be appreciated :)
> 
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@fromorbit.com