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@xxxxxxxxx> 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@xxxxxxxxx wrote: >>>>>>> From: Ira Weiny <ira.weiny@xxxxxxxxx> >>>>>>> >>>>>>> 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@xxxxxxxxx> >>>>>>> --- >>>>>>> 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 >