linux-kernel - Re: [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter

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Date:   Fri, 27 Aug 2021 18:53:24 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>, cluster-devel@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into
 fault_in_iov_iter_readable

On Fri, Aug 27, 2021 at 06:49:11PM +0200, Andreas Gruenbacher wrote:
> Turn iov_iter_fault_in_readable into a function that returns the number
> of bytes not faulted in (similar to copy_to_user) instead of returning a
> non-zero value when any of the requested pages couldn't be faulted in.
> This supports the existing users that require all pages to be faulted in
> as well as new users that are happy if any pages can be faulted in at
> all.
> 
> Rename iov_iter_fault_in_readable to fault_in_iov_iter_readable to make
> sure that this change doesn't silently break things.

I really disagree with these calling conventions.  "Number not faulted in"
is bloody useless; make it "nothing could be faulted in"/"something had
been faulted in" and it would make sense.  Failure several pages into the
area should not be treated as a hard error, for one thing, and ANY user
of that thing will have to cope with short copies anyway, no matter how
much you've managed to fault in.