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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 9 Sep 2021 15:56:09 -0700
Message-ID: <CAHk-=wi2fJ1XrgkfSYgn9atCzmJZ8J3HO5wnPO0Fvh5rQx9mmA@mail.gmail.com>
Subject: Re: [git pull] iov_iter fixes
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Pavel Begunkov <asml.silence@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Thu, Sep 9, 2021 at 3:21 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> On 9/9/21 3:56 PM, Linus Torvalds wrote:
> >
> > IOW, can't we have  that
> >
> >         ret = io_iter_do_read(req, iter);
> >
> > return partial success - and if XFS does that "update iovec on
> > failure", I could easily see that same code - or something else -
> > having done the exact same thing.
> >
> > Put another way: if the iovec isn't guaranteed to be coherent when an
> > actual error occurs, then why would it be guaranteed to be coherent
> > with a partial success value?
> >
> > Because in most cases - I'd argue pretty much all - those "partial
> > success" cases are *exactly* the same as the error cases, it's just
> > that we had a loop and one or more iterations succeeded before it hit
> > the error case.
>
> Right, which is why the reset would be nice, but reexpand + revert at
> least works and accomplishes the same even if it doesn't look as pretty.

You miss my point.

The partial success case seems to do the wrong thing.

Or am I misreading things? Lookie here, in io_read():

        ret = io_iter_do_read(req, iter);

let's say that something succeeds partially, does X bytes, and returns
a positive X.

The if-statements following it then do not trigger:

        if (ret == -EAGAIN || (req->flags & REQ_F_REISSUE)) {
  .. not this case ..
        } else if (ret == -EIOCBQUEUED) {
  .. nor this ..
        } else if (ret <= 0 || ret == io_size || !force_nonblock ||
                   (req->flags & REQ_F_NOWAIT) || !(req->flags & REQ_F_ISREG)) {
  .. nor this ..
        }

so nothing has been done to the iovec at all.

Then it does

        ret2 = io_setup_async_rw(req, iovec, inline_vecs, iter, true);

using that iovec that has *not* been reset, even though it really
should have been reset to "X bytes read".

See what I'm trying to say?

                Linus