Re: [PATCH v3 06/12] fs/xfs: Check if the inode supports DAX under lock

Dave Chinner
2020-02-11T06:16:49Z

On Sat, Feb 08, 2020 at 11:34:39AM -0800, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
>
> One of the checks for an inode supporting DAX is if the inode is
> reflinked. During a non-DAX to DAX state change we could race with
> the file being reflinked and end up with a reflinked file being in DAX
> state.
>
> Prevent this race by checking for DAX support under the MMAP_LOCK.

The on disk inode flags are protected by the XFS_ILOCK, not the
MMAP_LOCK. i.e. the MMAPLOCK provides data access serialisation, not
metadata modification serialisation.

>
> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> ---
> fs/xfs/xfs_ioctl.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> index da1eb2bdb386..4ff402fd6636 100644
> --- a/fs/xfs/xfs_ioctl.c
> +++ b/fs/xfs/xfs_ioctl.c
> @@ -1194,10 +1194,6 @@ xfs_ioctl_setattr_dax_invalidate(
>
> *join_flags = 0;
>
> - if ((fa->fsx_xflags & FS_XFLAG_DAX) == FS_XFLAG_DAX &&
> - !xfs_inode_supports_dax(ip))
> - return -EINVAL;
> -
> /* If the DAX state is not changing, we have nothing to do here. */
> if ((fa->fsx_xflags & FS_XFLAG_DAX) &&
> (ip->i_d.di_flags2 & XFS_DIFLAG2_DAX))
> @@ -1211,6 +1207,13 @@ xfs_ioctl_setattr_dax_invalidate(
>
> /* lock, flush and invalidate mapping in preparation for flag change */
> xfs_ilock(ip, XFS_MMAPLOCK_EXCL | XFS_IOLOCK_EXCL);
> +
> + if ((fa->fsx_xflags & FS_XFLAG_DAX) == FS_XFLAG_DAX &&
> + !xfs_inode_supports_dax(ip)) {
> + error = -EINVAL;
> + goto out_unlock;
> + }

Yes, you might be able to get away with reflink vs dax flag
serialisation on the inode flag modification, but it is not correct and
leaves a landmine for future inode flag modifications that are done
without holding either the MMAP or IOLOCK.

e.g. concurrent calls to xfs_ioctl_setattr() setting/clearing flags
other than the on disk DAX flag are all serialised by the ILOCK_EXCL
and will no be serialised against this DAX check. Hence if there are
other flags that we add in future that affect the result of
xfs_inode_supports_dax(), this code will not be correctly
serialised.

This raciness in checking the DAX flags is the reason that
xfs_ioctl_setattr_xflags() redoes all the reflink vs dax checks once
it's called under the XFS_ILOCK_EXCL during the actual change
transaction....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com