On Wed, Feb 21, 2018 at 10:15:24AM +1100, Dave Chinner wrote:
> On Tue, Feb 20, 2018 at 03:01:09PM -0800, Darrick J. Wong wrote:
> > On Sun, Feb 18, 2018 at 11:23:17AM +1100, Dave Chinner wrote:
> > > On Fri, Feb 16, 2018 at 09:22:47AM -0800, Darrick J. Wong wrote:
> > > > On Fri, Feb 16, 2018 at 10:04:26AM -0700, Dave Jiang wrote:
> > > > > In a situation where the rt_dev is DAX and data_dev is not DAX, if the user
> > > > > requests to remove the realtime flag via ioctl we can no longer support DAX
> > > > > for that file. Dynamic changing of S_DAX on the inode is not supported due
> > > > > to various complications in the existing implementation. Therefore until we
> > > > > address the dynamic S_DAX change issues, we must disallow realtime flag
> > > > > being removed.
> > > > >
> > > > > Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>
> > > > > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > > > > ---
> > > > > fs/xfs/xfs_ioctl.c | 14 ++++++++++++++
> > > > > 1 file changed, 14 insertions(+)
> > > > >
> > > > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > > > > index 2c70a0a4f59f..edd97d527fe8 100644
> > > > > --- a/fs/xfs/xfs_ioctl.c
> > > > > +++ b/fs/xfs/xfs_ioctl.c
> > > > > @@ -1030,6 +1030,20 @@ xfs_ioctl_setattr_xflags(
> > > > > {
> > > > > struct xfs_mount *mp = ip->i_mount;
> > > > > uint64_t di_flags2;
> > > > > + struct inode *inode = VFS_I(ip);
> > > > > + struct super_block *sb = inode->i_sb;
> > > > > +
> > > > > + /*
> > > > > + * In the case that the inode is realtime, and we are trying to remove
> > > > > + * the realtime flag, and the rtdev supports DAX but the datadev does
> > > > > + * not support DAX, we can't allow the realtime flag to be removed
> > > > > + * since we do not support dynamic S_DAX flag removal yet.
> > > > > + */
> > > > > + if (XFS_IS_REALTIME_INODE(ip) &&
> > > > > + !(fa->fsx_xflags & FS_XFLAG_REALTIME) &&
> > > > > + bdev_dax_supported(mp->m_rtdev_targp->bt_bdev, sb->s_blocksize) &&
> > > > > + !bdev_dax_supported(mp->m_ddev_targp->bt_bdev, sb->s_blocksize))
> > > >
> > > > What happens here if we have a non-rt file that we're trying to turn
> > > > into an rt file and the data dev supports dax but not the rt dev?
> > > >
> > > > Changing the rt flag is only supported on files with no data blocks (no
> > > > extents, no delalloc blocks), so why can't we remove S_DAX from an empty
> > > > file? There aren't any memory mappings or page cache to get in the way,
> > > > correct?
> > >
> > > File size can be non-zero, so you can have DAX read-over-hole
> > > mappings present. I simply don't think it's safe to remove/add S_DAX
> > > flags via ioctls right now. If we have a DAX capable rtdev, then the
> > > only way we should allow rtdev+dax to be used right now is via the
> > > RT inherit bit on the dir that creates files in the rtdev right from
> > > the start. i.e. we can't set/remove the RT inode flag on an inode
> > > via ioctl if rtdev+dax is enabled until the whole dynamic S_DAX
> > > inode flag thing is resolved.
> >
> > Could we deal with the restriction that the DAX flag can't change
> > (whether by user ioctl or by toggling the rt flag) unless the file size
> > is zero? That adds another way setting/clearing the realtime flag can
> > fail, but at least it'd be the same EINVAL.
>
> I thought we still mmap a zero length file and get a page fault that
> returns a zeroed page? Or does that segv?
I think it segfaults, but let's see...
$ rm -rf /opt/b ; xfs_io -f -c 'mmap -rw 0 1m' -c 'mread 512 20' /opt/b
Bus error
$ rm -rf /opt/b ; xfs_io -f -c 'mmap -rw 0 1m' -c 'mwrite 512 20' /opt/b
Bus error
--D
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
> --
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