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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: io-uring <io-uring@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH 2/3] io_uring: use iov_iter state save/restore helpers
Date: Tue, 14 Sep 2021 11:45:02 -0700	[thread overview]
Message-ID: <CAHk-=wh6mGm0b7AnKNRzDO07nrdpCrvHtUQ=afTH6pZ2JiBpeQ@mail.gmail.com> (raw)
In-Reply-To: <20210914141750.261568-3-axboe@kernel.dk>

On Tue, Sep 14, 2021 at 7:18 AM Jens Axboe <axboe@kernel.dk> wrote:
>
>
> +       iov_iter_restore(iter, state);
> +
...
>                 rw->bytes_done += ret;
> +               iov_iter_advance(iter, ret);
> +               if (!iov_iter_count(iter))
> +                       break;
> +               iov_iter_save_state(iter, state);

Ok, so now you keep iovb_iter and the state always in sync by just
always resetting the iter back and then walking it forward explicitly
- and re-saving the state.

That seems safe, if potentially unnecessarily expensive.

I guess re-walking lots of iovec entries is actually very unlikely in
practice, so maybe this "stupid brute-force" model is the right one.

I do find the odd "use __state vs rw->state" to be very confusing,
though. Particularly in io_read(), where you do this:

+       iov_iter_restore(iter, state);
+
        ret2 = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
        if (ret2)
                return ret2;

        iovec = NULL;
        rw = req->async_data;
-       /* now use our persistent iterator, if we aren't already */
-       iter = &rw->iter;
+       /* now use our persistent iterator and state, if we aren't already */
+       if (iter != &rw->iter) {
+               iter = &rw->iter;
+               state = &rw->iter_state;
+       }

        do {
-               io_size -= ret;
                rw->bytes_done += ret;
+               iov_iter_advance(iter, ret);
+               if (!iov_iter_count(iter))
+                       break;
+               iov_iter_save_state(iter, state);


Note how it first does that iov_iter_restore() on iter/state, buit
then it *replaces&* the iter/state pointers, and then it does
iov_iter_advance() on the replacement ones.

I don't see how that could be right. You're doing iov_iter_advance()
on something else than the one you restored to the original values.

And if it is right, it's sure confusing as hell.

           Linus

  reply	other threads:[~2021-09-14 18:45 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-14 14:17 [PATCHSET v2 0/3] Add ability to save/restore iov_iter state Jens Axboe
2021-09-14 14:17 ` [PATCH 1/3] iov_iter: add helper to save " Jens Axboe
2021-09-14 14:17 ` [PATCH 2/3] io_uring: use iov_iter state save/restore helpers Jens Axboe
2021-09-14 18:45   ` Linus Torvalds [this message]
2021-09-14 19:37     ` Jens Axboe
2021-09-14 23:02       ` Jens Axboe
2021-09-14 14:17 ` [PATCH 3/3] Revert "iov_iter: track truncated size" Jens Axboe
  -- strict thread matches above, loose matches on Subject: below --
2021-09-15 16:29 [PATCHSET v3 0/3] Add ability to save/restore iov_iter state Jens Axboe
2021-09-15 16:29 ` [PATCH 2/3] io_uring: use iov_iter state save/restore helpers Jens Axboe
2021-09-10 18:25 [PATCHSET 0/3] Add ability to save/restore iov_iter state Jens Axboe
2021-09-10 18:25 ` [PATCH 2/3] io_uring: use iov_iter state save/restore helpers Jens Axboe

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