Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

On 4/1/22 2:40 AM, Miklos Szeredi wrote:
> On Wed, 30 Mar 2022 at 19:49, Jens Axboe <axboe@kernel.dk> wrote:
>>
>> 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'...
> 
> Yep, this works now.
> 
> Next issue:  will get ENFILE even though there are just 40 slots.
> When running as root, then it will get as far as invoking the OOM
> killer, which is really bad.
> 
> There's no leak, this apparently only happens when the worker doing
> the fputs can't keep up.  Simple solution:  do the fput() of the
> previous file synchronously with the open_direct operation; fput
> shouldn't be expensive...  Is there a reason why this wouldn't work?

I take it you're continually reusing those slots? If you have a test
case that'd be ideal. Agree that it sounds like we just need an
appropriate breather to allow fput/task_work to run. Or it could be the
deferral free of the fixed slot.

-- 
Jens Axboe