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

[Ira Weiny]

On Wed, Apr 15, 2020 at 12:03:07PM -0400, Theodore Y. Ts'o wrote:
> 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,

But they can have FS_XFLAG_DAX 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.

:-( agreed.

FWIW without these patches an inode which has encrypt or verity set is already
turning off DAX...  So we already have a '-o dax' flag which is not "always".

:-(

Unfortunately the 'always' designation kind of breaks semantically but it is
equal to the current mount option.

> 
> 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.

ok...

> (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.)

ok...

Ira