Re: [PATCH] xfs: add regression test for DAX mount option usage

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Sep 12, 2017 at 04:44:11PM +1000, Dave Chinner wrote:
> On Fri, Sep 08, 2017 at 03:21:53PM -0600, Ross Zwisler wrote:
> > This adds a regression test for the following kernel patch:
> > 
> >   xfs: always use DAX if mount option is used
> > 
> > This test will also pass with kernel v4.14-rc1 and beyond because the XFS
> > DAX I/O mount option has been disabled (but not removed), so the
> > "chattr -x" to turn off DAX doesn't actually do anything.
> > 
> > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> > Suggested-by: Christoph Hellwig <hch@infradead.org>
> .....
> > +pgsize=`$here/src/feature -s`
> > +
> > +# disable tracing, clear the existing trace buffer and turn on dax tracepoints
> > +echo 0 > /sys/kernel/debug/tracing/tracing_on
> > +echo > /sys/kernel/debug/tracing/trace
> > +echo 1 > /sys/kernel/debug/tracing/events/fs_dax/enable
> > +
> > +# enable tracing for our one mmap I/O, then see if dax was used
> > +echo 1 > /sys/kernel/debug/tracing/tracing_on
> > +xfs_io  -t -c "truncate $pgsize" \
> > +	-c "chattr -x" \
> > +	-c "mmap -r 0 $pgsize" -c "mread 0 $pgsize" -c "munmap" \
> > +	-f $SCRATCH_MNT/testfile >> $seqres.full
> > +echo 0 > /sys/kernel/debug/tracing/tracing_on
> 
> So what happens when the user is already tracing the test to
> find a bug and the test turns all their tracing off?
> 
> Regardless of this screwing up developer bug triage, do we really
> want to add a dependency on kernel tracing into the test harness?

Yep, these are both valid concerns.  I ended up trying to get around both of
them by using perf instead in my v2, as suggested by Dan:

https://www.spinics.net/lists/linux-xfs/msg10420.html