From: Pavel Begunkov <asml.silence@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Al Viro <viro@zeniv.linux.org.uk>, Jens Axboe <axboe@kernel.dk>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [git pull] iov_iter fixes
Date: Thu, 9 Sep 2021 23:57:44 +0100 [thread overview]
Message-ID: <4ba3123b-6437-74b8-8205-1466065b8252@gmail.com> (raw)
In-Reply-To: <84c85780-fe43-e95b-312d-b7671c65a7aa@gmail.com>
On 9/9/21 11:54 PM, Pavel Begunkov wrote:
> On 9/9/21 8:37 PM, Linus Torvalds wrote:
>> On Wed, Sep 8, 2021 at 9:24 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>>>
>>> Fixes for io-uring handling of iov_iter reexpands
>>
>> Ugh.
>>
>> I have pulled this, because I understand what it does and I agree it
>> fixes a bug, but it really feels very very hacky and wrong to me.
>
> Maybe was worded not too clearly, my apologies.
>
>
>> It really smells like io-uring is doing a "iov_iter_revert()" using a
>> number that it pulls incorrectly out of its arse.
>
> It's not invented by io_uring,
>
> filemap.c : generic_file_direct_[write,read]()
>
> do the same thing. Also, the block layer was not re-expanding before
> ~5.12, so it looks it was possible to trigger a similar thing without
> io_uring, but I haven't tried to reproduce. Was mentioned in the
> cover-letter.
>
>> So when io-uring does that
>>
>> iov_iter_revert(iter, io_size - iov_iter_count(iter));
>>
>> what it *really* wants to do is just basically "iov_iter_reset(iter)".
>>
>> And that's basically what that addition of that "iov_iter_reexpand()"
>> tries to effectively do.
>>
>> Wouldn't it be better to have a function that does exactly that?
>>
>> Alternatively (and I'm cc'ing Jens) is is not possible for the
>> io-uring code to know how many bytes it *actually* used, rather than
>> saying that "ok, the iter originally had X bytes, now it has Y bytes,
>> so it must have used X-Y bytes" which was actively wrong for the case
>> where something ended up truncating the IO for some reason.
>>
>> Because I note that io-uring does that
>>
>> /* may have left rw->iter inconsistent on -EIOCBQUEUED */
>> iov_iter_revert(&rw->iter, req->result - iov_iter_count(&rw->iter));
>>
>> in io_resubmit_prep() too, and that you guys missed that it's the
>> exact same issue, and needs that exact same iov_iter_reexpand().
>
> Right. It was covered by v1-v2, which were failing requests with
> additional fallback in v2 [1], but I dropped in v3 [2] because there
> is a difference. Namely io_resubmit_prep() might be called deeply down
> the stack, e.g. in the block layer.
>
> It was intended to get fixed once the first part is merged, and I do
> believe that was the right approach, because there were certain
> communication delays. The first version was posted a month ago, but
> we missed the merged window. It appeared to me that if we get anything
> more complex
Dammit, apologies for the teared email.
... It was intended to get fixed once the first part is merged, and I do
believe that was the right approach, because there were certain
communication delays. The first version was posted a month ago, but
we missed the merged window. It appeared to me that if anything
more complex is posted, it would take another window to get it done.
> [1] https://lkml.org/lkml/2021/8/12/620
> [2] https://lkml.org/lkml/2021/8/23/285
>
>>
>> That "req->result" is once again the *original* length, and the above
>> code once again mis-handles the case of "oh, the iov got truncated
>> because of some IO limit".
>>
>> So I've pulled this, but I think it is
>>
>> (a) ugly nasty
>>
>> (b) incomplete and misses a case
>>
>> and needs more thought. At the VERY least it needs that
>> iov_iter_reexpand() in io_resubmit_prep() too, I think.
>>
>> I'd like the comments expanded too. In particular that
>>
>> /* some cases will consume bytes even on error returns */
>>
>> really should expand on the "some cases" thing, and why such an error
>> isn't fatal buye should be retried asynchronously blindly like this?
>>
>> Because I think _that_ is part of the fundamental issue here - the
>> io_uring code tries to just blindly re-submit the whole thing, and it
>> does it very badly and actually incorrectly.
>>
>> Or am I missing something?
>>
>> Linus
>>
>
--
Pavel Begunkov
next prev parent reply other threads:[~2021-09-09 22:58 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-09 4:22 [git pull] iov_iter fixes Al Viro
2021-09-09 19:37 ` Linus Torvalds
2021-09-09 21:19 ` Jens Axboe
2021-09-09 21:39 ` Jens Axboe
2021-09-09 21:56 ` Linus Torvalds
2021-09-09 22:21 ` Jens Axboe
2021-09-09 22:56 ` Linus Torvalds
2021-09-10 1:35 ` Jens Axboe
2021-09-10 2:43 ` Jens Axboe
2021-09-10 2:48 ` Al Viro
2021-09-10 3:06 ` Jens Axboe
2021-09-10 3:15 ` Al Viro
2021-09-10 3:23 ` Jens Axboe
2021-09-10 3:24 ` Al Viro
2021-09-10 3:28 ` Jens Axboe
2021-09-13 15:29 ` David Laight
2021-09-09 21:42 ` Dave Chinner
2021-09-10 2:57 ` Al Viro
2021-09-10 3:05 ` Jens Axboe
2021-09-10 3:11 ` Al Viro
2021-09-10 3:22 ` Jens Axboe
2021-09-10 3:27 ` Al Viro
2021-09-10 3:30 ` Jens Axboe
2021-09-10 3:36 ` Al Viro
2021-09-10 13:57 ` Jens Axboe
2021-09-10 14:42 ` Al Viro
2021-09-10 15:08 ` Jens Axboe
2021-09-10 15:32 ` Al Viro
2021-09-10 15:36 ` Jens Axboe
2021-09-10 15:04 ` Jens Axboe
2021-09-10 16:06 ` Jens Axboe
2021-09-10 16:44 ` Linus Torvalds
2021-09-10 16:56 ` Al Viro
2021-09-10 16:58 ` Linus Torvalds
2021-09-10 17:26 ` Jens Axboe
2021-09-10 17:31 ` Linus Torvalds
2021-09-10 17:32 ` Jens Axboe
2021-09-10 18:48 ` Al Viro
2021-09-10 19:04 ` Linus Torvalds
2021-09-10 19:10 ` Linus Torvalds
2021-09-10 19:10 ` Jens Axboe
2021-09-10 17:04 ` Jens Axboe
2021-09-09 22:54 ` Pavel Begunkov
2021-09-09 22:57 ` Pavel Begunkov [this message]
2021-09-09 23:14 ` Pavel Begunkov
2021-09-09 20:03 ` pr-tracker-bot
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