Re: [PATCH v6 3/3] xfs: reject removal of realtime flag when datadev doesn't support DAX

[Dave Jiang <dave.jiang@intel.com>]

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 <dave.jiang@intel.com>
>>>>>> Reviewed-by: Christoph Hellwig <hch@lst.de>
>>>>>> ---
>>>>>>  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?