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From:   Ian Lance Taylor <iant@golang.org>
Date:   Sun, 14 Feb 2021 17:12:47 -0800
Message-ID: <CAOyqgcX6HrbPU39nznmRMXJXtMWA0giYNRsio1jt1p5OU1jvOA@mail.gmail.com>
Subject: Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content
 is generated
To:     Dave Chinner <david@fromorbit.com>
Cc:     "Darrick J. Wong" <djwong@kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Nicolas Boichat <drinkcat@chromium.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Luis Lozano <llozano@chromium.org>,
        linux-fsdevel@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>
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On Sun, Feb 14, 2021 at 4:38 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Fri, Feb 12, 2021 at 03:54:48PM -0800, Darrick J. Wong wrote:
> > On Sat, Feb 13, 2021 at 10:27:26AM +1100, Dave Chinner wrote:
> >
> > > If you can't tell from userspace that a file has data in it other
> > > than by calling read() on it, then you can't use cfr on it.
> >
> > I don't know how to do that, Dave. :)
>
> If stat returns a non-zero size, then userspace knows it has at
> least that much data in it, whether it be zeros or previously
> written data. cfr will copy that data. The special zero length
> regular pipe files fail this simple "how much data is there to copy
> in this file" check...

This suggests that if the Go standard library sees that
copy_file_range reads zero bytes, we should assume that it failed, and
should use the fallback path as though copy_file_range returned
EOPNOTSUPP or EINVAL.  This will cause an extra system call for an
empty file, but as long as copy_file_range does not return an error
for cases that it does not support we are going to need an extra
system call anyhow.

Does that seem like a possible mitigation?  That is, are there cases
where copy_file_range will fail to do a correct copy, and will return
success, and will not return zero?

Ian