Re: [PATCHSET v3 0/4] fs: Support for LOOKUP_NONBLOCK / RESOLVE_NONBLOCK (Insufficiently faking current?)

[Jens Axboe <axboe@kernel.dk>]

On 2/15/21 7:41 PM, Jens Axboe wrote:
> On 2/15/21 3:41 PM, Eric W. Biederman wrote:
>> Jens Axboe <axboe@kernel.dk> writes:
>>
>>> On 2/15/21 11:24 AM, Jens Axboe wrote:
>>>> On 2/15/21 11:07 AM, Eric W. Biederman wrote:
>>>>> Linus Torvalds <torvalds@linux-foundation.org> writes:
>>>>>
>>>>>> On Sun, Feb 14, 2021 at 8:38 AM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>
>>>>>>>> Similarly it looks like opening of "/dev/tty" fails to
>>>>>>>> return the tty of the caller but instead fails because
>>>>>>>> io-wq threads don't have a tty.
>>>>>>>
>>>>>>> I've got a patch queued up for 5.12 that clears ->fs and ->files for the
>>>>>>> thread if not explicitly inherited, and I'm working on similarly
>>>>>>> proactively catching these cases that could potentially be problematic.
>>>>>>
>>>>>> Well, the /dev/tty case still needs fixing somehow.
>>>>>>
>>>>>> Opening /dev/tty actually depends on current->signal, and if it is
>>>>>> NULL it will fall back on the first VT console instead (I think).
>>>>>>
>>>>>> I wonder if it should do the same thing /proc/self does..
>>>>>
>>>>> Would there be any downside of making the io-wq kernel threads be per
>>>>> process instead of per user?
>>>>>
>>>>> I can see a lower probability of a thread already existing.  Are there
>>>>> other downsides I am missing?
>>>>>
>>>>> The upside would be that all of the issues of have we copied enough
>>>>> should go away, as the io-wq thread would then behave like another user
>>>>> space thread.  To handle posix setresuid() and friends it looks like
>>>>> current_cred would need to be copied but I can't think of anything else.
>>>>
>>>> I really like that idea. Do we currently have a way of creating a thread
>>>> internally, akin to what would happen if the same task did pthread_create?
>>>> That'd ensure that we have everything we need, without actively needing to
>>>> map the request types, or find future issues of "we also need this bit".
>>>> It'd work fine for the 'need new worker' case too, if one goes to sleep.
>>>> We'd just 'fork' off that child.
>>>>
>>>> Would require some restructuring of io-wq, but at the end of it, it'd
>>>> be a simpler solution.
>>>
>>> I was intrigued enough that I tried to wire this up. If we can pull this
>>> off, then it would take a great weight off my shoulders as there would
>>> be no more worries on identity.
>>>
>>> Here's a branch that's got a set of patches that actually work, though
>>> it's a bit of a hack in spots. Notes:
>>>
>>> - Forked worker initially crashed, since it's an actual user thread and
>>>   bombed on deref of kernel structures. Expectedly. That's what the
>>>   horrible kernel_clone_args->io_wq hack is working around for now.
>>>   Obviously not the final solution, but helped move things along so
>>>   I could actually test this.
>>>
>>> - Shared io-wq helpers need indexing for task, right now this isn't
>>>   done. But that's not hard to do.
>>>
>>> - Idle thread reaping isn't done yet, so they persist until the
>>>   context goes away.
>>>
>>> - task_work fallback needs a bit of love. Currently we fallback to
>>>   the io-wq manager thread for handling that, but a) manager is gone,
>>>   and b) the new workers are now threads and go away as well when
>>>   the original task goes away. None of the three fallback sites need
>>>   task context, so likely solution here is just punt it to system_wq.
>>>   Not the hot path, obviously, we're exiting.
>>>
>>> - Personality registration is broken, it's just Good Enough to compile.
>>>
>>> Probably a few more items that escape me right now. As long as you
>>> don't hit the fallback cases, it appears to work fine for me. And
>>> the diffstat is pretty good to:
>>>
>>>  fs/io-wq.c                 | 418 +++++++++++--------------------------
>>>  fs/io-wq.h                 |  10 +-
>>>  fs/io_uring.c              | 314 +++-------------------------
>>>  fs/proc/self.c             |   7 -
>>>  fs/proc/thread_self.c      |   7 -
>>>  include/linux/io_uring.h   |  19 --
>>>  include/linux/sched.h      |   3 +
>>>  include/linux/sched/task.h |   1 +
>>>  kernel/fork.c              |   2 +
>>>  9 files changed, 161 insertions(+), 620 deletions(-)
>>>
>>> as it gets rid of _all_ the 'grab this or that piece' that we're
>>> tracking.
>>>
>>> WIP series here:
>>>
>>> https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-worker
>>
>> I took a quick look through the code and in general it seems reasonable.
> 
> Great, thanks for checking.

Cleaner series here:

https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-worker.v2

One question, since I'm a bit stumped. The very top most debug patch:

https://git.kernel.dk/cgit/linux-block/commit/?h=io_uring-worker.v2&id=8a422f030b9630d16d5ec1ff97842a265f88485e

any idea what is going on here? For some reason, it only happens for
the 'manager' thread. That one doesn't do any work by itself, it's just
tasked with forking a new worker, if we need one.

-- 
Jens Axboe