Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

On 3/30/22 9:53 AM, Jens Axboe wrote:
> On 3/30/22 9:17 AM, Jens Axboe wrote:
>> On 3/30/22 9:12 AM, Miklos Szeredi wrote:
>>> On Wed, 30 Mar 2022 at 17:05, Jens Axboe <axboe@kernel.dk> wrote:
>>>>
>>>> On 3/30/22 8:58 AM, Miklos Szeredi wrote:
>>>>> Next issue:  seems like file slot reuse is not working correctly.
>>>>> Attached program compares reads using io_uring with plain reads of
>>>>> proc files.
>>>>>
>>>>> In the below example it is using two slots alternately but the number
>>>>> of slots does not seem to matter, read is apparently always using a
>>>>> stale file (the prior one to the most recent open on that slot).  See
>>>>> how the sizes of the files lag by two lines:
>>>>>
>>>>> root@kvm:~# ./procreads
>>>>> procreads: /proc/1/stat: ok (313)
>>>>> procreads: /proc/2/stat: ok (149)
>>>>> procreads: /proc/3/stat: read size mismatch 313/150
>>>>> procreads: /proc/4/stat: read size mismatch 149/154
>>>>> procreads: /proc/5/stat: read size mismatch 150/161
>>>>> procreads: /proc/6/stat: read size mismatch 154/171
>>>>> ...
>>>>>
>>>>> Any ideas?
>>>>
>>>> Didn't look at your code yet, but with the current tree, this is the
>>>> behavior when a fixed file is used:
>>>>
>>>> At prep time, if the slot is valid it is used. If it isn't valid,
>>>> assignment is deferred until the request is issued.
>>>>
>>>> Which granted is a bit weird. It means that if you do:
>>>>
>>>> <open fileA into slot 1, slot 1 currently unused><read slot 1>
>>>>
>>>> the read will read from fileA. But for:
>>>>
>>>> <open fileB into slot 1, slot 1 is fileA currently><read slot 1>
>>>>
>>>> since slot 1 is already valid at prep time for the read, the read will
>>>> be from fileA again.
>>>>
>>>> Is this what you are seeing? It's definitely a bit confusing, and the
>>>> only reason why I didn't change it is because it could potentially break
>>>> applications. Don't think there's a high risk of that, however, so may
>>>> indeed be worth it to just bite the bullet and the assignment is
>>>> consistent (eg always done from the perspective of the previous
>>>> dependent request having completed).
>>>>
>>>> Is this what you are seeing?
>>>
>>> Right, this explains it.   Then the only workaround would be to wait
>>> for the open to finish before submitting the read, but that would
>>> defeat the whole point of using io_uring for this purpose.
>>
>> Honestly, I think we should just change it during this round, making it
>> consistent with the "slot is unused" use case. The old use case is more
>> more of a "it happened to work" vs the newer consistent behavior of "we
>> always assign the file when execution starts on the request".
>>
>> Let me spin a patch, would be great if you could test.
> 
> Something like this on top of the current tree should work. Can you
> test?

You can also just re-pull for-5.18/io_uring, it has been updated. A last
minute edit make a 0 return from io_assign_file() which should've been
'true'...

-- 
Jens Axboe