Re: [PATCH RFC 4/8] fs/ext4: Introduce DAX inode flag

[Andreas Dilger <adilger@dilger.ca>]

We still need to store an on-disk DAX flag for Ext4, and at that point it
doesn't make sense not to expose it via the standard Ext4 chattr utility.

So having EXT4_DAX_FL (== FS_DAX_FL) is no extra effort to add.

Cheers, Andreas

> On Apr 16, 2020, at 20:20, Ira Weiny <ira.weiny@intel.com> wrote:
> 
> On Thu, Apr 16, 2020 at 06:57:31PM -0700, Darrick J. Wong wrote:
>>> On Thu, Apr 16, 2020 at 05:37:19PM -0700, Ira Weiny wrote:
>>> On Thu, Apr 16, 2020 at 03:49:37PM -0700, Darrick J. Wong wrote:
>>>> On Thu, Apr 16, 2020 at 03:33:27PM -0700, Ira Weiny wrote:
>>>>> On Thu, Apr 16, 2020 at 09:25:04AM -0700, Darrick J. Wong wrote:
>>>>>> On Mon, Apr 13, 2020 at 09:00:26PM -0700, ira.weiny@intel.com wrote:
>>>>>>> From: Ira Weiny <ira.weiny@intel.com>
>>>>>>> 
>>>>>>> Add a flag to preserve FS_XFLAG_DAX in the ext4 inode.
>>>>>>> 
>>>>>>> Set the flag to be user visible and changeable.  Set the flag to be
>>>>>>> inherited.  Allow applications to change the flag at any time.
>>>>>>> 
>>>>>>> Finally, on regular files, flag the inode to not be cached to facilitate
>>>>>>> changing S_DAX on the next creation of the inode.
>>>>>>> 
>>>>>>> Signed-off-by: Ira Weiny <ira.weiny@intel.com>
>>>>>>> ---
>>>>>>> fs/ext4/ext4.h  | 13 +++++++++----
>>>>>>> fs/ext4/ioctl.c | 21 ++++++++++++++++++++-
>>>>>>> 2 files changed, 29 insertions(+), 5 deletions(-)
>>>>>>> 
>>>>>>> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
>>>>>>> index 61b37a052052..434021fcec88 100644
>>>>>>> --- a/fs/ext4/ext4.h
>>>>>>> +++ b/fs/ext4/ext4.h
>>>>>>> @@ -415,13 +415,16 @@ struct flex_groups {
>>>>>>> #define EXT4_VERITY_FL            0x00100000 /* Verity protected inode */
>>>>>>> #define EXT4_EA_INODE_FL            0x00200000 /* Inode used for large EA */
>>>>>>> #define EXT4_EOFBLOCKS_FL        0x00400000 /* Blocks allocated beyond EOF */
>>>>>>> +
>>>>>>> +#define EXT4_DAX_FL            0x00800000 /* Inode is DAX */
>>>>>> 
>>>>>> Sooo, fun fact about ext4 vs. the world--
>>>>>> 
>>>>>> The GETFLAGS/SETFLAGS ioctl, since it came from ext2, shares the same
>>>>>> flag values as the ondisk inode flags in ext*.  Therefore, each of these
>>>>>> EXT4_[whatever]_FL values are supposed to have a FS_[whatever]_FL
>>>>>> equivalent in include/uapi/linux/fs.h.
>>>>> 
>>>>> Interesting...
>>>>> 
>>>>>> 
>>>>>> (Note that the "[whatever]" is a straight translation since the same
>>>>>> uapi header also defines the FS_XFLAG_[xfswhatever] flag values; ignore
>>>>>> those.)
>>>>>> 
>>>>>> Evidently, FS_NOCOW_FL already took 0x800000, but ext4.h was never
>>>>>> updated to note that the value was taken.  I think Ted might be inclined
>>>>>> to reserve the ondisk inode bit just in case ext4 ever does support copy
>>>>>> on write, though that's his call. :)
>>>>> 
>>>>> Seems like I should change this...  And I did not realize I was inherently
>>>>> changing a bit definition which was exposed to other FS's...
>>>> 
>>>> <nod> This whole thing is a mess, particularly now that we have two vfs
>>>> ioctls to set per-fs inode attributes, both of which were inherited from
>>>> other filesystems... :(
>>>> 
>>> 
>>> Ok I've changed it.
>>> 
>>>> 
>>>>>> 
>>>>>> Long story short - can you use 0x1000000 for this instead, and add the
>>>>>> corresponding value to the uapi fs.h?  I guess that also means that we
>>>>>> can change FS_XFLAG_DAX (in the form of FS_DAX_FL in FSSETFLAGS) after
>>>>>> that.
>>>>> 
>>>>> :-/
>>>>> 
>>>>> Are there any potential users of FS_XFLAG_DAX now?
>>>> 
>>>> Yes, it's in the userspace ABI so we can't get rid of it.
>>>> 
>>>> (FWIW there are several flags that exist in both FS_XFLAG_* and FS_*_FL
>>>> form.)
>>>> 
>>>>> From what it looks like, changing FS_XFLAG_DAX to FS_DAX_FL would be pretty
>>>>> straight forward.  Just to be sure, looks like XFS converts the FS_[xxx]_FL to
>>>>> FS_XFLAGS_[xxx] in xfs_merge_ioc_xflags()?  But it does not look like all the
>>>>> FS_[xxx]_FL flags are converted.  Is is that XFS does not support those
>>>>> options?  Or is it depending on the VFS layer for some of them?
>>>> 
>>>> XFS doesn't support most of the FS_*_FL flags.
>>> 
>>> If FS_XFLAG_DAX needs to continue to be user visible I think we need to keep
>>> that flag and we should not expose the EXT4_DAX_FL flag...
>>> 
>>> I think that works for XFS.
>>> 
>>> But for ext4 it looks like EXT4_FL_XFLAG_VISIBLE was intended to be used for
>>> [GET|SET]XATTR where EXT4_FL_USER_VISIBLE was intended to for [GET|SET]FLAGS...
>>> But if I don't add EXT4_DAX_FL in EXT4_FL_XFLAG_VISIBLE my test fails.
>>> 
>>> I've been playing with the flags and looking at the code and I _thought_ the
>>> following patch would ensure that FS_XFLAG_DAX is the only one visible but for
>>> some reason FS_XFLAG_DAX can't be set with this patch.  I still need the
>>> EXT4_FL_USER_VISIBLE mask altered...  Which I believe would expose EXT4_DAX_FL
>>> directly as well.
>>> 
>>> Jan, Ted?  Any ideas?  Or should we expose EXT4_DAX_FL and FS_XFLAG_DAX in
>>> ext4?
>> 
>> Both flags should be exposed through their respective ioctl interfaces
>> in both filesystems.  That way we don't have to add even more verbiage
>> to the documentation to instruct userspace programmers on how to special
>> case ext4 and XFS for the same piece of functionality.
> 
> Wouldn't it be more confusing for the user to have 2 different flags which do
> the same thing?
> 
> I would think that using FS_XFLAG_DAX _only_ (for both ext4 and xfs) would be
> easier without special cases?
> 
> Ira
>