Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 27 Aug 2021 19:23:49 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com
Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable
Message-ID: <YSk7xfcHVc7CxtQO@zeniv-ca.linux.org.uk>
References: <20210827164926.1726765-1-agruenba@redhat.com>
 <20210827164926.1726765-6-agruenba@redhat.com>
 <YSkz025ncjhyRmlB@zeniv-ca.linux.org.uk>
 <CAHk-=wh5p6zpgUUoY+O7e74X9BZyODhnsqvv=xqnTaLRNj3d_Q@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHk-=wh5p6zpgUUoY+O7e74X9BZyODhnsqvv=xqnTaLRNj3d_Q@mail.gmail.com>
Sender: Al Viro <viro@ftp.linux.org.uk>
Precedence: bulk

On Fri, Aug 27, 2021 at 12:05:32PM -0700, Linus Torvalds wrote:

> But see above. People *need* that ternary result, and "bytes/pages
> uncopied" is not only the traditional one we use elsewhere in similar
> situations, it's the one that has the easiest error tests for existing
> users (because zero remains "everything worked").

Could you show the cases where "partial copy, so it's OK" behaviour would
break anything?  

For that you would need the case where
	partial fault-in is currently rejected by the check
	checks downstream from there (for failing copy-in/copy-out) would
be either missing or would not be handled correctly in case of partial
fault-in or would slow a fast path down.

I don't see any such cases and I would be very surprised if such existed.
If you see any, please describe them - I could be wrong.  And I would
like to take a good look at any such case and see how well does it handle
possible short copy after full fault-in.