linux-kernel - Re: [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter offline CPUs

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Message-ID: <20230203163155.GA6734@willie-the-truck>
Date:   Fri, 3 Feb 2023 16:31:56 +0000
From:   Will Deacon <will@...nel.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH 1/2] cpuset: Fix cpuset_cpus_allowed() to not filter
 offline CPUs

On Thu, Feb 02, 2023 at 11:50:55AM -1000, Tejun Heo wrote:
> On Thu, Feb 02, 2023 at 04:05:14PM -0500, Waiman Long wrote:
> > 
> > On 2/2/23 15:53, Waiman Long wrote:
> > > 
> > > On 2/2/23 15:48, Tejun Heo wrote:
> > > > On Thu, Feb 02, 2023 at 03:46:02PM -0500, Waiman Long wrote:
> > > > > > > I will work on a patchset to do that as a counter offer.
> > > > > > We will need a small and simple patch for /urgent, or I will need to
> > > > > > revert all your patches -- your call.
> > > > > > 
> > > > > > I also don't tihnk you fully appreciate the ramifications of
> > > > > > task_cpu_possible_mask(), cpuset currently gets that quite wrong.
> > > > > OK, I don't realize the urgency of that. If it is that urgent, I
> > > > > will have
> > > > > no objection to get it in for now. We can improve it later on.
> > > > > So are you
> > > > > planning to get it into the current 6.2 rc or 6.3?
> > > > > 
> > > > > Tejun, are you OK with that as you are the cgroup maintainer?
> > > > Yeah, gotta fix the regression but is there currently a solution
> > > > which fixes
> > > > the regression but doesn't further break other stuff?
> > > 
> > > I believe there is a better way to do that, but it will need more time
> > > to flex out. Since cpuset_cpus_allowed() is only used by
> > > kernel/sched/core.c, Peter will be responsible if it somehow breaks
> > > other stuff.
> > 
> > Maybe my cpuset patch that don't update task's cpumask on cpu offline event
> > can help. However, I don't know the exact scenario where the regression
> > happen, so it may not.
> 
> Neither patch looks like they would break anything. That said, the patches
> aren't trivial and we're really close to the merge window, so I'd really
> appreciate if you can take a look and test a bit before we send these
> Linus's way. We can replace it with a better solution afterwards.

FWIW, I tested this series in an arm64 heterogeneous setup with things
like hotplug and exec()ing between 32-bit and 64-bit tasks and it all
seems good.

The alternative would be to revert Waiman's setaffinity changes, which
I've had a go at here:

https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=ssa-reverts

and I _think_ I've rescued the UAF fix too.

What do people prefer?

Will