linux-kernel - Re: [PATCH 2/9] fs/ext4: Disallow verity if inode is DAX

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Date:   Sun, 17 May 2020 22:32:07 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-ext4@...r.kernel.org,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        Al Viro <viro@...iv.linux.org.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] fs/ext4: Disallow verity if inode is DAX

On Fri, May 15, 2020 at 06:49:16PM -0700, Eric Biggers wrote:
> On Tue, May 12, 2020 at 10:43:17PM -0700, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > Verity and DAX are incompatible.  Changing the DAX mode due to a verity
> > flag change is wrong without a corresponding address_space_operations
> > update.
> > 
> > Make the 2 options mutually exclusive by returning an error if DAX was
> > set first.
> > 
> > (Setting DAX is already disabled if Verity is set first.)
> > 
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> > 
> > ---
> > Changes:
> > 	remove WARN_ON_ONCE
> > 	Add documentation for DAX/Verity exclusivity
> > ---
> >  Documentation/filesystems/ext4/verity.rst | 7 +++++++
> >  fs/ext4/verity.c                          | 3 +++
> >  2 files changed, 10 insertions(+)
> > 
> > diff --git a/Documentation/filesystems/ext4/verity.rst b/Documentation/filesystems/ext4/verity.rst
> > index 3e4c0ee0e068..51ab1aa17e59 100644
> > --- a/Documentation/filesystems/ext4/verity.rst
> > +++ b/Documentation/filesystems/ext4/verity.rst
> > @@ -39,3 +39,10 @@ is encrypted as well as the data itself.
> >  
> >  Verity files cannot have blocks allocated past the end of the verity
> >  metadata.
> > +
> > +Verity and DAX
> > +--------------
> > +
> > +Verity and DAX are not compatible and attempts to set both of these flags on a
> > +file will fail.
> > +
> 
> If you build the documentation, this shows up as its own subsection
> "2.13. Verity and DAX" alongside "2.12. Verity files", which looks odd.
> I think you should delete this new subsection header so that this paragraph goes
> in the existing "Verity files" subsection.

Ok...  I'll fix it up...

> 
> Also, Documentation/filesystems/fsverity.rst already mentions DAX (similar to
> fscrypt.rst).  Is it intentional that you added this to the ext4-specific
> documentation instead?

I proposed this text[1] and there were no objections...  I was looking at ext4
because only ext4 supports verity and DAX.  I think having this in both the
ext4 docs and the verity docs helps.

Ira

[1] https://lore.kernel.org/lkml/20200415191451.GA2305801@iweiny-DESK2.sc.intel.com/

> 
> - Eric