Re: [PATCH] common/rc: Fix _check_s_dax()

[Christoph Hellwig]

On Wed, Dec 02, 2020 at 01:41:45PM -0800, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> There is a conflict with the user visible statx bits 'mount root' and
> 'dax'.  The kernel is changing the dax bit to correct this conflict.[1]
> 
> Adjust _check_s_dax() to use the new bit.  Because DAX tests do not run
> on root mounts, STATX_ATTR_MOUNT_ROOT should always be 0, therefore we
> can allow either bit to indicate DAX and cover any kernel which may be
> running.
> 
> [1] https://lore.kernel.org/lkml/3e28d2c7-fbe5-298a-13ba-dcd8fd504666@xxxxxxxxxx/
> 
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> ---
> 
> I went ahead and used Christoph's suggestion regarding using both bits.

That wasn't my suggestion.  I think we should always error out when
the bit value shared with STATX_ATTR_MOUNT_ROOT is seen.  Because that
means the kernel is not using or fixed ABI we agreed to use going
forward.