linux-kernel - Re: [RFC][PATCHSET] iov

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Message-ID: <CAHk-=wjxkH79DcqVrZbETWERxLFU4xoPSzXkJOxfkxYKbjUaiw@mail.gmail.com>
Date:   Mon, 7 Jun 2021 16:35:46 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Sterba <dsterba@...e.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Anton Altaparmakov <anton@...era.com>,
        David Howells <dhowells@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [RFC][PATCHSET] iov_iter work

On Mon, Jun 7, 2021 at 3:01 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>  (b) on all the common non-SET_FS architectures, kernel threads using
> iov_iter_init() wouldn't work anyway, because on those architectures
> it would always fill the thing in with an iov, not a kvec.

Thinking more about this thing, I think it means that what we *should*
do is simply just

  void iov_iter_init(struct iov_iter *i, unsigned int direction,
                        const struct iovec *iov, unsigned long nr_segs,
                        size_t count)
  {
        WARN_ON_ONCE(direction & ~(READ | WRITE));
        iWARN_ON_ONCE(uaccess_kernel());
        *i = (struct iov_iter) {
                .iter_type = ITER_IOVEC,
                .data_source = direction,
                .iov = iov,
                .nr_segs = nr_segs,
                .iov_offset = 0,
                .count = count
        };
  }

because filling it with a kvec is simply wrong. It's wrong exactly due
to the fact that *if* we have a kernel thread, all the modern
non-SET_FS architectures will just ignore that entirely, and always
use the iov meaning.

So just do that WARN_ON_ONCE() to show that something is wrong (the
exact same way that the direction thing needs to be proper), and then
just fill it in as an ITER_IOVEC.

Because handling that legacy KERNEL_DS case as a KVEC is actively not
right anyway and doesn't match what a kernel thread would do on x86 or
arm64, so don't even try.

                 Linus