Re: [PATCH 04/10] fs: add infrastructure for multigrain timestamps

[Andi Kleen <ak@linux.intel.com>]

Jeff Layton <jlayton@kernel.org> writes:
>> 
>> Would it be of any benefit to keep a distinct ctime_floor in each
>> super block instead?
>> 
>
> Good question. Dave Chinner suggested the same thing, but I think it's
> a potential problem:
>
> The first series had to be reverted because inodes that had been
> modified in order could appear to be modified in reverse order with the
> right combination of fine and coarse grained timestamps. With the new
> floor value, that's no longer possible, but if we were to make it per-
> sb then it becomes possible again with files in different filesystems.
>
> This sort of timestamp comparison is done by tools like "make", and
> it's rather common to keep built objects in one location and generate
> or copy source files to another. My worry is that managing the floor as
> anything but a global value could cause regressions in those sorts of
> workloads.

Have you considered the interactions with time name spaces?

It seems you may need a floor for each, otherwise if the floor is set by
some name space with a more future time the other users lose out.

Also there's the issue to what happens if time gets set backwards.

-Andi