Re: [PATCH v5 0/9] fs: multigrain timestamp redux

[Josef Bacik]

On Thu, Jul 11, 2024 at 07:08:04AM -0400, Jeff Layton wrote:
> tl;dr for those who have been following along:
> 
> There are several changes in this version. The conversion of ctime to
> be a ktime_t value has been dropped, and we now use an unused bit in
> the nsec field as the QUERIED flag (like the earlier patchset did).
> 
> The floor value is now tracked as a monotonic clock value, and is
> converted to a realtime value on an as-needed basis. This eliminates the
> problem of trying to detect when the realtime clock jumps backward.
> 
> Longer patch description for those just joining in:
> 
> At LSF/MM this year, we had a discussion about the inode change
> attribute. At the time I mentioned that I thought I could salvage the
> multigrain timestamp work that had to be reverted last year [1].
> 
> That version had to be reverted because it was possible for a file to
> get a coarse grained timestamp that appeared to be earlier than another
> file that had recently gotten a fine-grained stamp.
> 
> This version corrects the problem by establishing a per-time_namespace
> ctime_floor value that should prevent this from occurring. In the above
> situation, the two files might end up with the same timestamp value, but
> they won't appear to have been modified in the wrong order.
> 
> That problem was discovered by the test-stat-time gnulib test. Note that
> that test still fails on multigrain timestamps, but that's because its
> method of determining the minimum delay that will show a timestamp
> change will no longer work with multigrain timestamps. I have a patch to
> change the testcase to use a different method that is in the process of
> being merged.
> 
> The testing I've done seems to show performance parity with multigrain
> timestamps enabled vs. disabled, but it's hard to rule this out
> regressing some workload.
> 
> This set is based on top of Christian's vfs.misc branch (which has the
> earlier change to track inode timestamps as discrete integers). If there
> are no major objections, I'd like to have this considered for v6.12,
> after a nice long full-cycle soak in linux-next.
> 
> PS: I took a stab at a conversion for bcachefs too, but it's not
> trivial. bcachefs handles timestamps backward from the way most
> block-based filesystems do. Instead of updating them in struct inode and
> eventually copying them to a disk-based representation, it does the
> reverse and updates the timestamps in its in-core image of the on-disk
> inode, and then copies that into struct inode. Either that will need to
> be changed, or we'll need to come up with a different way to do this for
> bcachefs.
> 
> [1]: https://lore.kernel.org/linux-fsdevel/20230807-mgctime-v7-0-d1dec143a704@xxxxxxxxxx/
> 
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>

Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx>

Thanks,

Josef