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

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Date:   Fri, 15 May 2020 18:49:16 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     ira.weiny@...el.com
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 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.

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?

- Eric