3:46 a.m.
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(a)intel.com>
>>>>> Reviewed-by: Christoph Hellwig <hch(a)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?