linux-kernel - Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP

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Date:   Mon, 31 Jul 2017 22:15:12 -0400
From:   Colin Walters <walters@...bum.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Dan Williams <dan.j.williams@...el.com>, Jan Kara <jack@...e.cz>,
        linux-nvdimm@...ts.01.org, Dave Chinner <david@...morbit.com>,
        linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
        Jeff Moyer <jmoyer@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP_IMMUTABLE

On Mon, Jul 31, 2017, at 02:23 PM, Darrick J. Wong wrote:

> I don't think F_SEAL_{SHRINK,GROW} prevents reflinking or CoW of file data,
> which are two things that cannot happen under S_IOMAP_IMMUTABLE that
> aren't size changes.  From the implementation it looks like shrink and
> grow are only supposed to disallow changes to i_size, not i_blocks (or
> the file block map).

True. 

> Then again, I suppose F_SEAL_* only work on shmem, so maybe it simply
> isn't defined for any other filesystem...?  e.g. it doesn't prohibit
> reflink, but the only fs implementing seals doesn't support reflink.
> 
> <shrug>
> 
> Seals cannot be removed, which is too strict for the S_IOMAP_IMMUTABLE
> user cases being presented.

To be clear, the set of use cases is swap files and DAX, right?  Or is there anything else?
I can't imagine why anyone would want to turn a swap file back into a regular file.
I haven't fully followed DAX, but I'd take your word for it if people want to
be able to remove the flag after.

Anyways, I think your broader point is right; the use cases are different enough
that it doesn't make sense to try to add S_CONTENT_IMMUTABLE (or however 
one decides to call it) at the same time.