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Date:   Fri, 17 Apr 2020 10:19:36 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH RFC 4/8] fs/ext4: Introduce DAX inode flag

On Fri, Apr 17, 2020 at 12:43:39AM -0600, Andreas Dilger wrote:
> 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.

I'll leave it exposed then.

Thanks,
Ira

> 
> Cheers, Andreas
> 
> > On Apr 16, 2020, at 20:20, Ira Weiny <ira.weiny@...el.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@...el.com wrote:
> >>>>>>> From: Ira Weiny <ira.weiny@...el.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@...el.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
> > 

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