Re: [PATCH] xfs: conditionally allow FS_XFLAG_REALTIME changes if S_DAX is set

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Aug 10, 2024 at 08:18:16PM +0530, Souptick Joarder wrote:
> On Tue, Aug 6, 2024 at 12:16 AM Darrick J. Wong <djwong@kernel.org> wrote:
> >
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > If a file has the S_DAX flag (aka fsdax access mode) set, we cannot
> > allow users to change the realtime flag unless the datadev and rtdev
> > both support fsdax access modes.  Even if there are no extents allocated
> > to the file, the setattr thread could be racing with another thread
> > that has already started down the write code paths.
> >
> > Fixes: ba23cba9b3bdc ("fs: allow per-device dax status checking for filesystems")
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> >  fs/xfs/xfs_ioctl.c |   11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > index 4e933db75b12..6b13666d4e96 100644
> > --- a/fs/xfs/xfs_ioctl.c
> > +++ b/fs/xfs/xfs_ioctl.c
> > @@ -483,6 +483,17 @@ xfs_ioctl_setattr_xflags(
> >                 /* Can't change realtime flag if any extents are allocated. */
> >                 if (ip->i_df.if_nextents || ip->i_delayed_blks)
> >                         return -EINVAL;
> > +
> > +               /*
> > +                * If S_DAX is enabled on this file, we can only switch the
> > +                * device if both support fsdax.  We can't update S_DAX because
> > +                * there might be other threads walking down the access paths.
> > +                */
> > +               if (IS_DAX(VFS_I(ip)) &&
> > +                   (mp->m_ddev_targp->bt_daxdev == NULL ||
> > +                    (mp->m_rtdev_targp &&
> > +                     mp->m_rtdev_targp->bt_daxdev == NULL)))
> > +                       return -EINVAL;
> Any chance of  mp->m_ddev_targp == NULL  ?

No, m_ddev_targp is the device you give to mount. e.g. if you do

# mount /dev/sda /mnt

then m_ddev_targp is the block_device /dev/sda.

--D

> >         }
> >
> >         if (rtflag) {
> >