linux-kernel - Re: [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in

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Date:   Sat, 24 Jul 2021 12:52:34 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andreas Gruenbacher <agruenba@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v4 1/8] iov_iter: Introduce iov_iter_fault_in_writeable helper

On Sat, Jul 24, 2021 at 12:35 PM Andreas Gruenbacher
<agruenba@...hat.com> wrote:
>
> +int iov_iter_fault_in_writeable(const struct iov_iter *i, size_t bytes)
> +{
...
> +                       if (fault_in_user_pages(start, len, true) != len)
> +                               return -EFAULT;

Looking at this once more, I think this is likely wrong.

Why?

Because any user can/should only care about at least *part* of the
area being writable.

Imagine that you're doing a large read. If the *first* page is
writable, you should still return the partial read, not -EFAULT.

So I think the code needs to return 0 if _any_ fault was successful.
Or perhaps return how much it was able to fault in. Because returning
-EFAULT if any of it failed seems wrong, and doesn't allow for partial
success being reported.

The other reaction I have is that you now only do the
iov_iter_fault_in_writeable, but then you make fault_in_user_pages()
still have that "bool write" argument.

We already have 'fault_in_pages_readable()', and that one is more
efficient (well, at least if the fault isn't needed it is). So it
would make more sense to just implement fault_in_pages_writable()
instead of that "fault_in_user_pages(, bool write)".

                 Linus