linux-ext4 - Re: Problems with determining data presence by examining extents?

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Date:   Wed, 15 Jan 2020 14:15:34 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     dhowells@...hat.com, linux-fsdevel@...r.kernel.org,
        viro@...iv.linux.org.uk, tytso@....edu, adilger.kernel@...ger.ca,
        darrick.wong@...cle.com, clm@...com, josef@...icpanda.com,
        dsterba@...e.com, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Problems with determining data presence by examining extents?

Christoph Hellwig <hch@....de> wrote:

> The whole idea of an out of band interface is going to be racy and suffer
> from implementation loss.  I think what you want is something similar to
> the NFSv4.2 READ_PLUS operation - give me that if there is any and
> otherwise tell me that there is a hole.  I think this could be a new
> RWF_NOHOLE or similar flag, just how to return the hole size would be
> a little awkward.  Maybe return a specific negative error code (ENODATA?)
> and advance the iov anyway.

Just having call_iter_read() return a short read could be made to suffice...
provided the filesystem doesn't return data I haven't written in (which could
cause apparent corruption) and does return data I have written in (otherwise I
have to go back to the server).

David