linux-ext4 - Re: [RFC 1/2] iomap: direct-io: Move inode_dio_begin before filemap_write_and_wait

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Date:   Wed, 15 Jan 2020 06:56:47 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [RFC 1/2] iomap: direct-io: Move inode_dio_begin before
 filemap_write_and_wait_range

On Wed, Jan 15, 2020 at 10:19:25AM +0100, Jan Kara wrote:
> On Tue 14-01-20 08:38:18, Christoph Hellwig wrote:
> > On Tue, Jan 14, 2020 at 10:05:07AM +0100, Jan Kara wrote:
> > > 
> > > Well, XFS always performs buffered writeback using unwritten extents so at
> > > least the immediate problem of stale data exposure ext4 has does not happen
> > > there AFAICT. 
> > 
> > Currently XFS never uses unwritten extents when converting delalloc
> > extents.
> 
> I see, it is a long time since I last looked at that part of XFS code. So
> then I think XFS might be prone to the same kind of race and data exposure
> as I outlined in [1]...

I think not using unwritten extents for filling holes inside i_size will
always lead to the potential for stale data exposure in one form or
another.  Because of that Darrick has started looking into always using
unwritten extents for buffered writes inside i_size.