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

From: Jens Axboe
Date: Sun Oct 27 2019 - 23:39:22 EST

On 10/27/19 1:59 PM, Pavel Begunkov wrote:
> 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.

That's always appreciated!

It struck me that while I've added quite a few regression tests, we don't
have any that just do basic read/write using the variety of settings we
have for that. So I added that to liburing.

Jens Axboe