Re: [PATCH 7/9] vhost: allow userspace to create workers

[Mike Christie <michael.christie@oracle.com>]

On 6/7/21 10:19 AM, Stefan Hajnoczi wrote:
> My concern is that threads should probably accounted against
> RLIMIT_NPROC and max_threads rather than something indirect like 128 *
> RLIMIT_NOFILE (a userspace process can only have RLIMIT_NOFILE
> vhost-user file descriptors open).
> 

Ah ok, I see what you want I think.

Ok, I think the options are:

0. Nothing. Just use existing indirect/RLIMIT_NOFILE.

1. Do something like io_uring's create_io_thread/copy_process. If we call
copy_process from the vhost ioctl context, then the userspace process that
did the ioctl will have it's processes count incremented and checked against
its rlimit.

The drawbacks:
- This gets a little more complicated than just calling copy_process though.
We end up duplicating a lot of the kthread API.
- We have to deal with new error cases like the parent exiting early.
- I think all devs sharing a worker have to have the same owner. kthread_use_mm
and kthread_unuse_mm to switch between mm's for differrent owner's devs seem to
be causing lots of errors. I'm still looking into this one though.

2.  It's not really what you want, but for unbound work io_uring has a check for
RLIMIT_NPROC in the io_uring code. It does:

wqe->acct[IO_WQ_ACCT_UNBOUND].max_workers =
					task_rlimit(current, RLIMIT_NPROC);

then does:

if (!ret && acct->nr_workers < acct->max_workers) {

Drawbacks:
In vhost.c, we could do something similar. It would make sure that vhost.c does
not create more worker threads than the rlimit value, but we wouldn't be
incrementing the userspace process's process count. The userspace process could
then create RLIMIT_NPROC threads and vhost.c could also create RLIMIT_NPROC
threads, so we end up with 2 * RLIMIT_NPROC threads.

3. Change the kthread and copy_process code so we can pass in the thread
(or it's creds or some struct that has the values that need to be check) that
needs to be checked and updated.

Drawback:
This might be considered too ugly for how special case vhost is. For example, we
need checks/code like the io_thread/PF_IO_WORKER code in copy_process for io_uring.
I can see how added that for io_uring because it affects so many users, but I can
see how vhost is not special enough.