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

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Date:   Sun, 27 Oct 2019 21:56:24 +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 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.

-- 
Yours sincerely,
Pavel Begunkov



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