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Date:   Mon, 1 Aug 2022 17:45:46 +0100
From:   Will Deacon <will@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] sched: Use user_cpus_ptr for saving user provided
 cpumask in sched_setaffinity()
Message-ID: <20220801164545.GA26471@willie-the-truck>
References: <20220801154124.2011987-1-longman@redhat.com>
 <20220801154124.2011987-2-longman@redhat.com>
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On Mon, Aug 01, 2022 at 11:41:23AM -0400, Waiman Long wrote:
> The user_cpus_ptr field is added by commit b90ca8badbd1 ("sched:
> Introduce task_struct::user_cpus_ptr to track requested affinity"). It
> is currently used only by arm64 arch due to possible asymmetric cpu
> setup. This patch extends its usage to save user provided cpumask when
> sched_setaffinity() is called for all arches.
> 
> To preserve the existing arm64 use case, a new cpus_affinity_set flag is
> added to differentiate if user_cpus_ptr is set up by sched_setaffinity()
> or by force_compatible_cpus_allowed_ptr(). user_cpus_ptr
> set by sched_setaffinity() has priority and won't be
> overwritten by force_compatible_cpus_allowed_ptr() or
> relax_compatible_cpus_allowed_ptr().
> 
> As a call to sched_setaffinity() will no longer clear user_cpus_ptr
> but set it instead, the SCA_USER flag is no longer necessary and can
> be removed.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  include/linux/sched.h |  1 +
>  kernel/sched/core.c   | 71 +++++++++++++++++++++++++++++++------------
>  kernel/sched/sched.h  |  1 -
>  3 files changed, 52 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index c46f3a63b758..60ae022fa842 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -815,6 +815,7 @@ struct task_struct {
>  
>  	unsigned int			policy;
>  	int				nr_cpus_allowed;
> +	int				cpus_affinity_set;
>  	const cpumask_t			*cpus_ptr;
>  	cpumask_t			*user_cpus_ptr;
>  	cpumask_t			cpus_mask;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index da0bf6fe9ecd..7757828c7422 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2607,6 +2607,7 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>  		return -ENOMEM;
>  
>  	cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> +	dst->cpus_affinity_set = src->cpus_affinity_set;

I haven't been through this thorougly, but it looks a bit suspicious to me
to inherit this field directly across fork(). If a 64-bit task with this
flag set forks and then exec's a 32-bit program, arm64 will be in trouble if
we're not able to override the affinity forcefully.

Will