Date:   Sun, 27 Oct 2019 22:17:34 +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 22:02, 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 

-- 
Yours sincerely,
Pavel Begunkov



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