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

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Message-ID: <57db9960-0b31-9f40-c13b-1db6dcc88920@gmail.com>
Date:   Sun, 27 Oct 2019 20:19:50 +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 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.

-- 
Yours sincerely,
Pavel Begunkov



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