linux-kernel - Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost

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Message-ID: <Z2RhNcJbP67CRqaM@kbusch-mbp.dhcp.thefacebook.com>
Date: Thu, 19 Dec 2024 11:08:53 -0700
From: Keith Busch <kbusch@...nel.org>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	michael.christie@...cle.com, Tejun Heo <tj@...nel.org>,
	Luca Boccassi <bluca@...ian.org>
Subject: Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost_task

On Thu, Dec 19, 2024 at 06:42:09PM +0100, Paolo Bonzini wrote:
> On 12/19/24 18:32, Keith Busch wrote:
> > This appears to be causing a user space regression. The library
> > "minijail" is used by virtual machine manager "crossvm". crossvm uses
> > minijail to fork processes, but the library requires the process be
> > single threaded. Prior to this patch, the process was single threaded,
> > but this change creates a relationship from the kvm thread to the user
> > process that fails minijail's test.
> 
> Thanks for the report.
> 
> The minijail code has a flag that's documented like this:
> 
>     /// Disables the check that prevents forking in a multithreaded environment.
>     /// This is only safe if the child process calls exec immediately after
>     /// forking. The state of locks, and whether or not they will unlock
>     /// is undefined. Additionally, objects allocated on other threads that
>     /// expect to be dropped when those threads cease execution will not be
>     /// dropped.
>     /// Thus, nothing should be called that relies on shared synchronization
>     /// primitives referenced outside of the current thread. The safest
>     /// way to use this is to immediately exec in the child.
> 
> Is crosvm trying to do anything but exec?  If not, it should probably use the
> flag.

Good point, and I'm not sure right now. I don't think I know any crosvm
developer experts but I'm working on that to get a better explanation of
what's happening,