From: Dave Jiang Subject: Re: [PATCH v6 3/3] xfs: reject removal of realtime flag when datadev doesn't support DAX Date: Tue, 27 Feb 2018 09:46:54 -0700 Message-ID: <1e6ed5f9-fdc1-f0f4-1ff7-e068f6506f39@intel.com> 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> <20180220231524.GD7000@dastard> <20180220232358.GH27629@magnolia> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org To: "Darrick J. Wong" , Dave Chinner Return-path: In-Reply-To: <20180220232358.GH27629@magnolia> Content-Language: en-US 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 02/20/2018 04:23 PM, Darrick J. Wong wrote: > 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 >>>>>> 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? > > 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 Darrick, So you want the change to be if the file size is 0 then we can modify the RT bit, otherwise reject if DAX is involved?