From: Dave Chinner Subject: Re: [PATCH v6 3/3] xfs: reject removal of realtime flag when datadev doesn't support DAX Date: Wed, 21 Feb 2018 10:15:24 +1100 Message-ID: <20180220231524.GD7000@dastard> References: <151880058592.43131.558230052922233871.stgit@djiang5-desk3.ch.intel.com> <151880066614.43131.15847174332875373188.stgit@djiang5-desk3.ch.intel.com> <20180216172247.GS5217@magnolia> <20180218002317.GU7000@dastard> <20180220230109.GG27629@magnolia> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Darrick J. Wong" Return-path: Content-Disposition: inline In-Reply-To: <20180220230109.GG27629@magnolia> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org 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 > > > > Reviewed-by: Christoph Hellwig > > > > --- > > > > 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? Cheers, Dave. -- Dave Chinner david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org