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Date:   Wed, 8 Dec 2021 16:13:27 -0600
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Subject: Re: [PATCH V6 01/10] Use copy_process in vhost layer
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To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     geert@linux-m68k.org, vverma@digitalocean.com, hdanton@sina.com,
        hch@infradead.org, stefanha@redhat.com, jasowang@redhat.com,
        sgarzare@redhat.com, virtualization@lists.linux-foundation.org,
        christian.brauner@ubuntu.com, axboe@kernel.dk,
        linux-kernel@vger.kernel.org
References: <20211129194707.5863-1-michael.christie@oracle.com>
 <20211208153420-mutt-send-email-mst@kernel.org>
From:   michael.christie@oracle.com
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On 12/8/21 2:34 PM, Michael S. Tsirkin wrote:
> On Mon, Nov 29, 2021 at 01:46:57PM -0600, Mike Christie wrote:
>> The following patches made over Linus's tree, allow the vhost layer to do
>> a copy_process on the thread that does the VHOST_SET_OWNER ioctl like how
>> io_uring does a copy_process against its userspace app. This allows the
>> vhost layer's worker threads to inherit cgroups, namespaces, address
>> space, etc and this worker thread will also be accounted for against that
>> owner/parent process's RLIMIT_NPROC limit.
>>
>> If you are not familiar with qemu and vhost here is more detailed
>> problem description:
>>
>> Qemu will create vhost devices in the kernel which perform network, SCSI,
>> etc IO and management operations from worker threads created by the
>> kthread API. Because the kthread API does a copy_process on the kthreadd
>> thread, the vhost layer has to use kthread_use_mm to access the Qemu
>> thread's memory and cgroup_attach_task_all to add itself to the Qemu
>> thread's cgroups.
>>
>> The problem with this approach is that we then have to add new functions/
>> args/functionality for every thing we want to inherit. I started doing
>> that here:
>>
>> https://urldefense.com/v3/__https://lkml.org/lkml/2021/6/23/1233__;!!ACWV5N9M2RV99hQ!ceUHd4m4MTJFOGccB9N5r7WonxVoYYT2XPiYwWt2-Vt1Y-DmQirRN8OqKozFLN1h73N6$ 
>>
>> for the RLIMIT_NPROC check, but it seems it might be easier to just
>> inherit everything from the beginning, becuase I'd need to do something
>> like that patch several times.
> 
> 
> So who's merging this? Me? Did all patches get acked by appropriate
> maintainers?
> 

Not yet.

Jens, The last open review comment was from you for the name change
and additional patch description info.

In this posting, I changed the name from:

kernel_worker/kernel_worker_start

to

user_worker_create/user_worker_start

I didn't do the start/create_user_worker format originally discussed
because a while back Christoph had given me a review comment about trying
to tie everything together into an API. Everything having the user_worker
prefix felt nicer in that it was easy to tell the functions and flags went
together, and so I thought it would handle his comment too.

And patch:

[PATCH V6 07/10] io_uring: switch to user_worker

should better explain the reason for the switch.