On Nov 3, 2016, at 2:43 PM, Theodore Ts'o [off-list ref] wrote:

On Thu, Nov 03, 2016 at 09:48:27AM +1100, Dave Chinner wrote:

quoted

We're going to need regression tests for this to ensure that it
works properly and that we don't inadvertantly break it in future.
Can you write some xfstests that exercise this functionality and
validate that the mount behaviour, clamping and range limiting is
working as intended?

In order to have automated regression tests which are file system
independent, we need a way to query what are the timestamps that a
particular mounted file systme supports.  One approach would be to use
fsinfo, which David Howells had been working on, but which has been
bike-shedded to death for the last n years, and I'd hate to block this
patch series behind a proposed new fsinfo(2) system call.

I wish we could just get the fsinfo and statx calls landed, but I agree
it would be a DOS to block any other patches waiting for that to land...

or maybe we _should_ block other patches behind that patch, and force it
to be landed... :-)

Alternatively, we can just create a specialized ioctl to return that
information which is non-ideal in other dimensions.

The last option, which is admittedly ugly, would be to create an shell
function which knows how to figure out the max_timestamp and
min_timestamp by using the file system name and querying the
superblock using dumpe2fs, xfs_db, etc.

I'd argue for the last option because once we do get a programmtic way
to get the information via a system call such as fsinfo(2), we can
convert xfstests to use it, where as if we add an ioctl to return this
information, we'll have to support the ioctl forever.

Does this make sense?   Any objections?

Realistically, there are only a handful of filesystems being tested by
xfstests, and it is simple enough to hard-code these limits into the
test script for ext4, xfs, btrfs, etc. since the limits will not be
changing very often (and it is noteworthy when they do).  If and when
there is an interface to query these values from the kernel, it may
still make sense to keep the hard-coded limits to verify the syscall
against...

Cheers, Andreas