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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 27 Aug 2021 12:05:32 -0700
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Subject: Re: [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable
To:     Al Viro <viro@zeniv.linux.org.uk>
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
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On Fri, Aug 27, 2021 at 11:52 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> Again, the calling conventions are wrong.  Make it success/failure or
> 0/-EFAULT.  And it's inconsistent for iovec and non-iovec cases as it is.

Al, the 0/-EFAULT thing DOES NOT WORK.

The whole "success vs failure" model is broken.

Because "success" for some people is "everything worked".

And for other people it is "at least _part_ of it worked".

So no, 0/-EFAULT fundamentally cannot work, because the return needs
to be able to handle that ternary situation (ie "nothing" vs
"something" vs "everything").

This is *literally* the exact same thing that we have for
copy_to/from_user(). And Andreas' solution (based on my suggestion) is
the exact same one that we have had for that code since basically day
#1.

The whole "0/-EFAULT" is simpler, yes. And it's what
"{get|put}_user()" uses, yes. And it's more common to a lot of other
functions that return zero or an error.

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").

Andreas originally had that "how many bytes/pages succeeded" return
value instead, and yes, that's also ternary. But it means that now the
common "complete success" test ends up being a lot uglier, and the
semantics of the function changes completely where "0" no longer means
success, and that messes up much more.

So I really think you are barking entirely up the wrong tree.

If there is any inconsistency, maybe we should make _more_ cases use
that "how many bytes/pages not copied" logic, but in a lot of cases
you don't actually need the ternary decision value.

So the inconsistency is EXACTLY the same as the one we have always had
for get|put_user() vs copy_to|from_user(), and it exists for the EXACT
same reason.

IOW, please explain how you'd solve the ternary problem without making
the code a lot uglier.

              Linus