linux-kernel - Re: [CFT][PATCH v3] fork, vhost: Use CLONE

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Date:   Mon, 5 Jun 2023 10:46:51 -0500
From:   Mike Christie <michael.christie@...cle.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>, linux@...mhuis.info,
        nicolas.dichtel@...nd.com, axboe@...nel.dk,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, jasowang@...hat.com, stefanha@...hat.com,
        brauner@...nel.org
Subject: Re: [CFT][PATCH v3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

On 6/5/23 10:10 AM, Oleg Nesterov wrote:
> On 06/03, michael.christie@...cle.com wrote:
>>
>> On 6/2/23 11:15 PM, Eric W. Biederman wrote:
>> The problem is that as part of the flush the drivers/vhost/scsi.c code
>> will wait for outstanding commands, because we can't free the device and
>> it's resources before the commands complete or we will hit the accessing
>> freed memory bug.
> 
> ignoring send-fd/clone issues, can we assume that the final fput/release
> should always come from vhost_worker's sub-thread (which shares mm/etc) ?

I think I'm misunderstanding the sub-thread term.

- Is it the task_struct's context that we did the
kernel/vhost_taskc.c:vhost_task_create() from? Below it would be the
thread we did VHOST_SET_OWNER from.

If so, then yes.

- Is it the task_struct that gets created by
kernel/vhost_taskc.c:vhost_task_create()?

If so, then the answer is no. vhost_task_create has set the no_files
arg on kernel_clone_args, so copy_files() sets task_struct->files to NULL
and we don't clone or dup the files.

So it works like if we were using a kthread still:

1. Userapce thread0 opens /dev/vhost-$something.
2. thread0 does VHOST_SET_OWNER ioctl. This calls vhost_task_create() to
create the task_struct which runs the vhost_worker() function which handles
the work->fns.
3. If userspace now does a SIGKILL or just exits without doing a close() on
/dev/vhost-$something, then when thread0 does exit_files() that will do the
fput that does vhost-$something's file_operations->release.