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From: Jens Axboe <axboe@kernel.dk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: io-uring <io-uring@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCHSET 0/3] Add ability to save/restore iov_iter state
Date: Mon, 13 Sep 2021 19:54:56 -0600	[thread overview]
Message-ID: <fc6649ca-7700-a8ca-2e37-6f93c8aadb4d@kernel.dk> (raw)
In-Reply-To: <CAHk-=wj3Lu=mJ8L7iE0RQXGZVdoSMz6rnPmrWoVNJhTaObOqkA@mail.gmail.com>

On 9/13/21 5:23 PM, Linus Torvalds wrote:
> On Mon, Sep 13, 2021 at 3:43 PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> Al, Linus, are you OK with this? I think we should get this in for 5.15.
>> I didn't resend the whole series, just a v2 of patch 1/3 to fix that bvec
>> vs iovec issue. Let me know if you want the while thing resent.
> 
> So I'm ok with the iov_iter side, but the io_uring side seems still
> positively buggy, and very confused.
> 
> It also messes with the state in bad ways and has internal knowledge.
> And some of it looks completely bogus.
> 
> For example, I see
> 
>         state->count -= ret;
>         rw->bytes_done += ret;
> 
> and I go "that's BS". There's no way it's ok to start messing with the
> byte count inside the state like that. That just means that the state
> is now no longer the saved state, and it's some random garbage.
>
> I also think that the "bytes_done += ret" is a big hint there: any
> time you restore the iovec state, you should then forward it by
> "bytes_done". But that's not what the code does.
> 
> Instead, it will now restore the iovec styate with the *wrong* number
> of bytes remaining, but will start from the beginning of the iovec.
> 
> So I think the fs/io_uring.c use of this state buffer is completely wrong.
> 
> What *may* be the right thing to do is to
> 
>  (a) not mess with state->count
> 
>  (b) when you restore the state you always use
> 
>         iov_iter_restore(iter, state, bytes_done);
> 
> to actually restore the *correct* state.
> 
> Because modifying the iovec save state like that cannot be right, and
> if it's right it's still too ugly and fragile for words. That save
> state should be treated as a snapshot, not as a random buffer that you
> can make arbitrary changes to.
> 
> See what I'm saying?

OK, for the do while loop itself, I do think we should be more
consistent and that would also get rid of the state->count manipulation.
I do agree that messing with that state is not something that should be
done, and we can do this more cleanly and consistently instead. Once we
hit the do {} while loop, state should be &rw->state and we can
consistently handle it that way.

Let me rework that bit and run the tests, and I'll post a v2 tomorrow.
Thanks for taking a closer look.

-- 
Jens Axboe


      reply	other threads:[~2021-09-14  1:54 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-10 18:25 Jens Axboe
2021-09-10 18:25 ` [PATCH 1/3] iov_iter: add helper to save " Jens Axboe
2021-09-10 18:50   ` Al Viro
2021-09-10 19:15     ` [PATCH v2 " Jens Axboe
2021-09-10 18:25 ` [PATCH 2/3] io_uring: use iov_iter state save/restore helpers Jens Axboe
2021-09-10 18:25 ` [PATCH 3/3] Revert "iov_iter: track truncated size" Jens Axboe
2021-09-13 22:43 ` [PATCHSET 0/3] Add ability to save/restore iov_iter state Jens Axboe
2021-09-13 23:23   ` Linus Torvalds
2021-09-14  1:54     ` Jens Axboe [this message]

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