linux-kernel - Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

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Date:   Sat, 27 May 2023 09:12:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Mike Christie <michael.christie@...cle.com>,
        linux@...mhuis.info, nicolas.dichtel@...nd.com, axboe@...nel.dk,
        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: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

On Sat, May 27, 2023 at 2:49 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> The real sticky widget for me is how to handle one of these processes
> coredumping.  It really looks like it will result in a reliable hang.

Well, if *that* is the main worry, I think that's trivial enough to deal with.

In particular, we could make the rule just be that user worker threads
simply do not participate in core-dumps.

THAT isn't hard.

All we need to do is

 (a) not count those threads in zap_threads()

 (b) make sure that they don't add themselves to the "dumper" list by
not calling "coredujmp_task_exit()"

 (c) not initiate core-dumping themselves.

and I think that's pretty much it.

In fact, that really seems like a good model *regardless*, because
honestly, a PF_IO_WORKER doesn't have valid register state for the
core dump anyway, and anything that would have caused a IO thread to
get a SIGSEGV *should* have caused a kernel oops already.

So the only worry is that the core dump will now happen while an IO
worker is still busy and so it's not "atomic" wrt possible VM changes,
but while that used to be a big problem back in the dark ages when we
didn't get the VM locks for core dumping, that got fixed a few years
ago because it already caused lots of potential issues.

End result: I think the attached patch is probably missing something,
but the approach "FeelsRight(tm)" to me.

Comments?

                   Linus

View attachment "patch.diff" of type "text/x-patch" (2133 bytes)