linux-kernel - Re: [PATCH 0/2][for-next] cleanup submission path

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Message-ID: <1e0ae1a6-6b8b-a78e-8ec0-fb7aa5972d00@gmail.com>
Date:   Sun, 27 Oct 2019 22:59:13 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2][for-next] cleanup submission path

On 27/10/2019 22:51, Jens Axboe wrote:
> On 10/27/19 1:17 PM, Pavel Begunkov wrote:
>> On 27/10/2019 22:02, Jens Axboe wrote:
>>> On 10/27/19 12:56 PM, Pavel Begunkov wrote:
>>>> On 27/10/2019 20:26, Jens Axboe wrote:
>>>>> On 10/27/19 11:19 AM, Pavel Begunkov wrote:
>>>>>> On 27/10/2019 19:56, Jens Axboe wrote:
>>>>>>> On 10/27/19 10:49 AM, Jens Axboe wrote:
>>>>>>>> On 10/27/19 10:44 AM, Pavel Begunkov wrote:
>>>>>>>>> On 27/10/2019 19:32, Jens Axboe wrote:
>>>>>>>>>> On 10/27/19 9:35 AM, Pavel Begunkov wrote:
>>>>>>>>>>> A small cleanup of very similar but diverged io_submit_sqes() and
>>>>>>>>>>> io_ring_submit()
>>>>>>>>>>>
>>>>>>>>>>> Pavel Begunkov (2):
>>>>>>>>>>>         io_uring: handle mm_fault outside of submission
>>>>>>>>>>>         io_uring: merge io_submit_sqes and io_ring_submit
>>>>>>>>>>>
>>>>>>>>>>>        fs/io_uring.c | 116 ++++++++++++++------------------------------------
>>>>>>>>>>>        1 file changed, 33 insertions(+), 83 deletions(-)
>>>>>>>>>>
>>>>>>>>>> I like the cleanups here, but one thing that seems off is the
>>>>>>>>>> assumption that io_sq_thread() always needs to grab the mm. If
>>>>>>>>>> the sqes processed are just READ/WRITE_FIXED, then it never needs
>>>>>>>>>> to grab the mm.
>>>>>>>>>> Yeah, we removed it to fix bugs. Personally, I think it would be
>>>>>>>>> clearer to do lazy grabbing conditionally, rather than have two
>>>>>>>>> functions. And in this case it's easier to do after merging.
>>>>>>>>>
>>>>>>>>> Do you prefer to return it back first?
>>>>>>>>
>>>>>>>> Ah I see, no I don't care about that.
>>>>>>>
>>>>>>> OK, looked at the post-patches state. It's still not correct. You are
>>>>>>> grabbing the mm from io_sq_thread() unconditionally. We should not do
>>>>>>> that, only if the sqes we need to submit need mm context.
>>>>>>>
>>>>>> That's what my question to the fix was about :)
>>>>>> 1. Then, what the case it could fail?
>>>>>> 2. Is it ok to hold it while polling? It could keep it for quite
>>>>>> a long time if host is swift, e.g. submit->poll->submit->poll-> ...
>>>>>>
>>>>>> Anyway, I will add it back and resend the patchset.
>>>>>
>>>>> If possible in a simple way, I'd prefer if we do it as a prep patch and
>>>>> then queue that up for 5.4 since we now lost that optimization.  Then
>>>>> layer the other 2 on top of that, since I'll just rebase the 5.5 stuff
>>>>> on top of that.
>>>>>
>>>>> If not trivially possible for 5.4, then we'll just have to leave with it
>>>>> in that release. For that case, you can fold the change in with these
>>>>> two patches.
>>>>>
>>>> Hmm, what's the semantics? I think we should fail only those who need
>>>> mm, but can't get it. The alternative is to fail all subsequent after
>>>> the first mm_fault.
>>>
>>> For the sqthread setup, there's no notion of "do this many". It just
>>> grabs whatever it can and issues it. This means that the mm assign
>>> is really per-sqe. What we did before, with the batching, just optimized
>>> it so we'd only grab it for one batch IFF at least one sqe in that batch
>>> needed the mm.
>>>
>>> Since you've killed the batching, I think the logic should be something
>>> ala:
>>>
>>> if (io_sqe_needs_user(sqe) && !cur_mm)) {
>>> 	if (already_attempted_mmget_and_failed_ {
>>> 		-EFAULT end sqe
>>> 	} else {
>>> 		do mm_get and mmuse dance
>>> 	}
>>> }
>>>
>>> Hence if the sqe doesn't need the mm, doesn't matter if we previously
>>> failed. If we need the mm and previously failed, -EFAULT.
>>>
>> That makes sense, but a bit hard to implement honoring links and drains
> 
> If it becomes too complicated or convoluted, just drop it. It's not
> worth spending that much time on.
> 
I've already done it more or less elegantly, just prefer to test commits
before sending.

-- 
Yours sincerely,
Pavel Begunkov



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