Re: [PATCH RFC 3/8] fs/ext4: Disallow encryption if inode is DAX

[Theodore Y. Ts'o]

On Mon, Apr 13, 2020 at 09:00:25PM -0700, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> Encryption and DAX are incompatible.  Changing the DAX mode due to a
> change in Encryption mode is wrong without a corresponding
> address_space_operations update.
> 
> Make the 2 options mutually exclusive by returning an error if DAX was
> set first.
> 
> Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>

The encryption flag is inherited from the containing directory, and
directories can't have the DAX flag set, so anything we do in
ext4_set_context() will be safety belt / sanity checking in nature.

But we *do* need to figure out what we do with mount -o dax=always
when the file system might have encrypted files.  My previous comments
about the verity flag and dax flag applies here.

Also note that encrypted files are read/write so we must never allow
the combination of ENCRPYT_FL and DAX_FL.  So that may be something
where we should teach __ext4_iget() to check for this, and declare the
file system as corrupted if it sees this combination.  (For VERITY_FL
&& DAX_FL that is a combo that we might want to support in the future,
so that's probably a case where arguably, we should just ignore the
DAX_FL for now.)

					- Ted