Re: [fstests PATCH v2] xfs: add regression test for DAX mount option usage

[Eryu Guan <eguan@redhat.com>]

Hi Ross,

On Mon, Sep 11, 2017 at 02:01:03PM -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>
> ---
> 
> Changes since v1:
>  - Use perf instead of tracepoints to detect whether DAX is used. (Dan)

Thanks for the test! But I agreed with Dave here, it doesn't seem like a
good idea to depend on the kernel tracepoints in a test, but I can't
think of a better solution either, so I didn't get to this patch
earlier..

Before XFS disabled the ability to switch on & off per-inode DAX flag,
the x flag was only shown after an explicit 'chattr +x', even if XFS was
mounted with dax option, e.g.

# mkfs -t xfs -f /dev/ram0
# mount -o dax /dev/ram0 /mnt/xfs
# echo "test" > /mnt/xfs/testfile
# xfs_io -c "lsattr" /mnt/xfs/testfile
---------------- /mnt/xfs/testfile
# xfs_io -c "chattr +x" /mnt/xfs/testfile
# xfs_io -c "lsattr" /mnt/xfs/testfile
---------------x /mnt/xfs/testfile

I'm wondering if it makes sense to make lsattr print the x flag by
default when XFS is mounted with dax option, that way, we have a method
to know whether dax is used or not on a particular file too.

Thanks,
Eryu